There isn't a one-book-you-have-to-read on colour theory I don't think, but Gurney's book comes pretty close.
Presume you saw the other current thread with the plugs of the Handprint site? For years now I've recommended that for anyone interested in the subject since it's 1, an excellent distillation of a lot of disparate sources, with a lot of very worthwhile insights into the subject from the author himself, and 2, free. Hard to beat that.
Einion