In terms of historical interest, Cromwell's protectorate is a poor relation to the English bourgeois revolution. As Ted Valance points out, the Protectorate“has too often been dismissed as merely a dreary Puritanical prelude to the restoration of monarchy”.
Having said that Providence Lost does go a little way in rectifying this anomaly. The book is a well written and researched and is a solid albeit conservative piece of historical study.
Paul lay is a gifted writer. His book is mostly narrative-driven but does contain some useful insights, but even his skill and experience cannot make the Protectorate as dramatic as the English revolution.
The book covers the years 1653 to 1659. The Protectorate replaced the short-lived English republic and replaced it with the dictatorship of the Major Generals with Cromwell at its head.
While Lay's book is not without analysis,but it would be better for the use of work of the great social historian as Lay calls Christopher Hil. Hill succinctly describes the social, political and economic processes that“ dissolved the Long Parliament forcibly in 1653, nominated a convention of his own adherents (the Barebones Parliament), which revived the social and economic demands of the petty bourgeoisie and had to be hastily dissolved. Cromwell was then proclaimed Protector under a Constitution (the Instrument of Government”), which was rigged to conceal the dictatorship of the Army officers. He called a Parliament under this constitution on a new £200 franchise, by which moneyed men were admitted to vote and the lesser freeholders excluded. But Parliament and Army quarrelled, Parliament was dissolved, and a period of naked military dictatorship followed under the Major-Generals, in which the Cavaliers were finally disarmed. Ultimately Cromwell and his Court circle (representing especially the new civil service), under pressure from the City, came to realise that the Army had done its job and that its maintenance now meant a crushing burden of taxation on the propertied classes, for which no compensating advantages were obtained”.
Once Cromwell had crushed all opposition to his and the army's rule, he gave the bourgeoisie free rein to launch their conquests both at home and abroad. The short term failure of the conquests abroad gave Lay his book title.
The launching of Cromwell's imperial enterprises gave a massive impulse to the early capitalist class. It is, therefore, no accident that perhaps the most dubious and money-grabbing members of the petty bourgeoisie were chosen to lead the attempted overseas conquests. In doing so they marked the end of the “Good Old Cause” leading Edward Burrough, later to lament “where is the good old cause now?...and what is become of it? In whose hands does it lie? 
Like many aspects of the English revolution to the Protectorate is open to many interpretations not all historians agree that there was a naked military dictatorship, some like Austin Woolrych tend to downplay the extent of the dictatorship.
Although not a historian in the strictest sense the Russian Marxist Leon Trotsky took a different view in that “Under Cromwell's leadership the revolution acquired all the breadth vital for it. In such cases as that of the Levellers, where it exceeded the bounds of the requirements of the regenerate bourgeois society, Cromwell ruthlessly put down the “Lunaticks.” Once victorious, Cromwell began to construct a new state law that coupled biblical texts with the lances of the 'holy' soldiers, under which the deciding word always belonged to the pikes. On 19th April 1653, Cromwell broke up the rump of the Long Parliament. In recognition of his historical mission, the Puritan dictator saw dispersed members on the way with biblical denunciations: “Thou drunkard!” he cried to one; “Thou adulterer!” he reminded another. After this Cromwell forms a parliament out of representatives of God-fearing people, that is, an essentially class parliament; the God-fearers were the middle class who completed the work of accumulation with the aid of strict morality and set about the plunder of the whole world with the Holy Scriptures on their lips”.
Trotsky had a specific economic and class approach. This type of approach is mostly absent from Lay's book. The book is mainly devoid of economic analysis, especially the economic base of the Major Generals. When he does mention other historians, they are mostly conservative in nature. Christopher Hill is only mentioned once as a footnote. Hill did not write much on this subject, but when he did, it was worth reading, especially his early work.
His study of some of the Russian historians in work such as Soviet Interpretations of the English Interregnum was groundbreaking. Hill takes a far more historical materialist approach to the Protectorate in that he saw historical developments in class terms as this quote from his essay Soviet Interpretations of the English Interregnum highlights "There were rifts within classes as well as between them. The squires particularly occupied a double position-they were hostile to the old order but were themselves landlords, interested in enclosing, in keeping the peasantry in their place. This accounts for their re-alliance with the defeated Cavaliers after they had attained their ends: there was a common enemy to fear. Their fear of further social revolution held the squires back from completely finishing with the old landed order and so destroying the economic roots of the monarchy. 1659 marks a series of desperate attempts to conserve the republic without social reforms. But none of the groups that won power could reunite the interests of their class as a whole. Charles II was the most satisfactory heir to Oliver Cromwell if he was prepared to accept the revolution. And that is what the declaration of Breda did by referring all questions to parliament. " Let the king come in," Harrington had said, " and call a parliament of the greatest cavaliers in England, so they be men of estates, and let them sit but seven years and they will all turn commonwealth's men”. 
One question Lay's book does not satisfactorily answer is why did so much power end up in so few hands? While Lay would not be caught dead using Leon Trotsky and Hill for political reasons could not, Trotsky is well worth reading for his clear-sighted analysis on the class nature of Cromwell's rule. In his essay Two traditions: the seventeenth-century revolution and Chartism he shows that "Different classes in different conditions and for different tasks find themselves compelled in particular and indeed, the most acute and critical periods in their history, to vest an extraordinary power and authority in much of their leaders as can carry forward their fundamental interests most sharply and fully". Cromwell's Protectorate was one such example. "For one era Oliver Cromwell, and for another, Robespierre expressed the historically progressive tendencies of development of bourgeois society." 
As Lay points out the foreign policy of Cromwell would set the template for British capitalism imperialist conquests for centuries to come. The future empire, as was the protectorate, would be built on slavery.
A significant part of Cromwell's foreign policy or Western Design” was based on writings of a dubious former Dominican friar, Thomas Gage. Gage himself writes, “I humbly pray your Excellency [Cromwell] . . . direct your noble thoughts to employ the soldiery of this Kingdom upon such just an honorable design in those parts of America. The fact that Cromwell took advice from Gage was a severe miscalculation that would end in disaster and humiliation for the English bourgeoisie. It is perhaps fitting that Cage met his end dying of dysentery upon arrival in Jamaica.
The failure of the western Design” caused significant problems for the Cromwell and his generals. A regime based on military power is only as good as its last victory. The defeat of Cromwell’s army and navy by the Spanish despite having numerical superiority caused Cromwell to increase his power at the expense of the already narrow franchise. Thus began the Rule of the Major-Generals”.
This “military dictatorship" carried out a campaign of intimidation and brutality that would have been unthinkable even under the Monarchy.Parliament's brutal punishment of the Quaker James Nayler in 1656 (his tongue was bored with a red-hot iron) for imitating Christ's entry in Jerusalem was one such episode. As Rowan Williams writes in his book review, this was one of only many cases of “judicial sadism.”
The debacle at the hands of the Spanish also led to the escalation of attacks by class forces hostile to the Cromwell regime. A spate of Royalist/Leveller plots and assassination attempts were only foiled because of Cromwell's vast spy network under the leadership of John Thurloe.
Lay's book has been reviewed by substantial sections of both big and small media with the majority praising the book. The book is attractive to review because of its conservative historiography. Lay somewhat unusually presents very little in the way of a conclusion at the end of the book.
Despite Lay being a trustee of the Cromwell Museum even a cursory read of the book tells us that Lay only goes so far with his admiration for Oliver Cromwell. He certainly feels ill at ease with the revolutionary nature of the Cromwellian revolution. Lay expresses horror at Cromwell's and other regicides killing of the King and believes the act was illegal.
Lay's hostility to the revolutionary acts carried out by the English bourgeoisie is mirrored by Rowan Williams former Archbishop of Canterbury who in his review of Lay's book said “It is a pattern familiar in revolutionary narratives: the point at which it ought, at last, to be obvious to all that the revolution was right and justified when one could be confident of being on the side of history, keeps slipping over the horizon. Someone must be blamed, and the revolution inexorably descends into factional warfare. In recent decades, analysis by political thinkers such as Raymond Williams and Gillian Rose has stressed the inevitability of “long revolutions”, and the dangers of messianic end-of-history aspirations and the bloodshed that accompanies them. This should remind us of the foolishness of speaking as though history had “sides”, yet still the left and the right resort to such defaults. Lay’s book sheds light on this process, despite the fact that Cromwell’s must have been, historically, one of the least terror-ridden of revolutions”. 
Whether you could call Lay, a Whig historian is open to debate. His book does have a whiff of the Whig interpretation of history. As revolutions go you get the feeling that Lay would be more at home with the “Glorious Revolution” when James II quit his throne and his kingdom overnight, and William of Orange was installed as king. This was the kind of palace revolution that Lay would prefer rather than the 1640 revolution.
To conclude my criticisms of Lay should not put off anyone who would like to know more about the subject. Readers also should not be put off by any of significant criticisms from reviewers of Lay's conservative approach to history.
Lay probably feels the Protectorate was a failure, and with the restoration of the monarchy, not much had really changed. But as Trotsky said “Charles II swung Cromwell's corpse upon the gallows. But pre-Cromwellian society could not be re-established by any restoration. The works of Cromwell could not be liquidated by the thievish legislation of the Restoration because what has been written with the sword cannot be wiped out by the pen”.
 ] The English Revolution 1640-https://www.marxists.org/archive/hill-christopher/english-revolution/
 To the Whole English Army(1659)
 Commonwealth to Protectorate-by Austin Woolrych
 Two traditions: the seventeenth-century revolution and Chartism- https://www.marxists.org/archive/trotsky/works/britain/ch06.htm
 Soviet Interpretations of the English Interregnum-Christopher Hill-The Economic History Review, Vol. 8, No. 2 (May, 1938), pp. 159-167
 Two traditions: the seventeenth-century revolution and Chartism
 For further information on the life of Cage see https://johnjburnslibrary.wordpress.com/2013/09/16/the-many-faces-of-thomas-gage/
 Oliver Cromwell, the man who wouldn’t be king-https://www.newstatesman.com/culture/books/2020/01/oliver-cromwell-man-who-wouldn-t-be-king