Plenty of "free" info out there on the web, youtube, etc. these days. But the best explanation of the Corner 5 system - what most people refer to as "The" diamond system, even though there are dozens if not hundreds - is in Byrne's Standard Book of Pool and Billiards. That's how I learned by chance, and after all of the other info I've seen on it, that section of the book remains the most well laid out and accurate on the subject. And the rest of the book is of course excellent as well, well worth it.
Scott