The best book I know on music theory is The Songwriting Secrets of the Beatles. A big wonderful book, especially if you have Beatle songs in your head -- the author uses the songs as examples of how theory works, and he is one of the great explainers. The problem is that the book is impossible to get in North America (I had to send away for it).

There is a truly fine article on Counterpoint in the most recent New Grove Dictionary, if you have access to a university library. It gives you the history, the methods, etc.   It is ridiculously technical in spots, but you can skip those.

yours,

Peter T.