Crossovers in comics books are cool. I remember reading Secret Wars from Marvel as a kid and thinking it was the coolest thing ever. I mean, here in one book were all of my favorite heroes (Spider-Man, Wolverine, Hulk, Fantastic Four) and their villains (Dr. Doom, Kang, Lizard, etc.). At the time, I didn't know that it was the first major crossover series ever (preceded only by the 3-issue Contest of Champions series). Somehow, I missed out on Crisis on Infinite Earths from DC, but I did pick up the revamped Superman that resulted from that series.

Anyway, for all the Comic Book Crossover information you can stand, I would recommend the The Unofficial Comics Crossover Index. It's filled with most of the major, cross-title events from the 1980's onward and only lacks the major events from the past few years. Best of all, each crossover is reviewed and lists all the related issues. While, much of the info may be on Wikipedia, it's nice to have all of it in one place and in one format.