This novel by Ronald Basset was first published in 1966 and then reissued with a new cover in 1968 as a tie-in with Michael Reeves' film of the same name. Besides being a interesting curiosity in its own right, it's both worth a read and provides some insights into the adaptation process undertaken by Reeves and his collaborators.
The first difference that is apparent, as mentioned by David Pirie in his seminal A Heritage of Horror, is that the film-makers dispense with much of the psychological motivation underlying the self-styled witch-finder of all England's actions. In the book he's the original forty year old virgin, regarding women with a mixture of fear and loathing.
In keeping with this, the book, which is divided up into sections covering the years 1643 to 1947, starts with Hopkins and his assistant-to-be Stearne as reluctant pikemen in the Parliamentarian army. They soon desert and embark on a more profitable career as witch-finders. These initial excursions are absent from the film, which begins with Hopkins secure in his position and, as such, showing little sign of insecurity.
Running parallel to this narrative we are also introduced to the young soldier Ralph Margery, also in the Parliamentarian army but a bit more dedicated to that cause than his enemies-to-be.
The two men's paths cross, as in the film, when Hopkins executes John Lowes, a clergyman suspected of Catholic sympathies. While Stearn simply has his way with Lowes's ward, Sara, Hopkins makes a bargain with her for the life of her uncle.
Margery swears that he will have his revenge on Hopkins for defiling his fiancee, and pursues the witchfinder as best he can, given the contingencies of the war raging around them.
There's a strong western element to all this, strangely reminiscent of the picaresque structure of The Good, The Bad and The Ugly, as the characters just manage to evade one another or, on occasion, unwittingly cross one another's paths.
At the same time the distinction between the resources available to Leone and to Reeves is highlighted by another omission from the film, that of the battle of Naseby where Hopkins is pressed into military service for a second time. Another omission, that of an orgy which turns out to be attended by the local magistrate upon whom Hopkins would otherwise depend upon for a successful prosecution, can presumably be attributed more to the censorship regime of the day.
The novel also has a different ending to the film, one that takes place in 1647 at a point by which the war has been won by the Parliamentarians but the peace has not yet been established. Compared to the film's unforgettable ending – Margery, consumed by his desire for revenge, hacks at Hopkins until one of his colleages puts the witchfinder out of his misery – it's somewhat anticlimactic.