The novelisation of Close Encounters of the Third Kind, because I've never got round to seeing the film and thought I might find it less hassle to read a book than rent a video. The downside, of course, is that there's a good chance it'll have the awful writing so common in film novelisations, which was why I was uncertain; I certainly wouldn't have bought it if it had cost more than the 50p I paid.

(Er, except that in fact I paid a pound, due to the second copy. Oops.)