Not an exact template for the movie, Ritual by David Pinner was, however, the book that inspired the movie – and it is getting a reprint in the very near future.
Rather than being set in the Scottish highlands, this 1967 novel is set in Cornwall, with police officer, David Hanlin asked to investigate the murder of a local child. Whilst there, he's subjected to mental trickery, seduction, ancient religious practices and sacrificial rituals. Sound familiar?
The rights to the book were obtained for the Wicker Man movie a few years later, with a fairly minimal rewrite creating the ultimate movie cult classic. Original copies go for silly money, so the £6.39 price tag for the new version, which comes with original cover artwork and a new forward by Saint Etienne's Bob Stanley, looks a serious bargain. Alternatively, a hardback version sells for £9.09.
I’m glad that this book is finally getting a re-release. I recall waiting months to borrow a copy from Camden library about ten years ago, seemingly the only copy in any local library, and although it is quite different to the film in terms of plot, it’s worth a read. Reasonable price too.