This is often regarded as a precursor Stevenson’s better-known Dr Jekyll and Mr Hyde, but I think it’s the better book. Considering how early it is, Confessions of a Justified Sinner is an amazingly sophisticated bit of storytelling with two narrators, one of them obviously insane, and a lot of ambiguity surrounding identity. One one level it’s a gripping story about two brothers, one good, the other evil, brought up in the strict Scottish Calvinist tradition with its disturbing belief in predestination. On the other hand it’s a supernatural murder mystery which leaves the reader to do a great deal of the work. The second half, the actual ‘confession’ itself, is one of the most disturbing things I’ve ever read. Weirdly, it’s never been made into a decent film, although Wikipedia tells me that Ian Rankin has written a screenplay and is trying to get it made.