The Wicker Man screenplay was based on a novel by David Pinner called Ritual. And a bit of inspiration from Sir J. G. Frazer's The Golden Bough.

After the film was completed, Hardy and Schaffer (the film's director and screenplay writer) collaborated on a novelization. It does indeed explain a lot of what the film leaves out, but mainly because the film version most people have seen had a lot of the filmed scenes edited out! So the book resembles the film as originally written/intended.