If you are referring to the standard three cushion system, I learned it from Robert Byrne's Standard Book of Pool and Billiards way back when, he explains it well and takes the time to mention the necessary adjustments etc. Takes a little adjusting from there to make it work on a pool table - I'm actually going to start covering this in my next few articles, check them out on go4pool.net (small commercial...).
If you are looking for other "diamond" systems, there are dozens for 1, 2, 3, and more rail kicking systems, also lot of information out there these days on various DVDs and books, several mentioned already...
Scott