I just reread The Thinking Man's Guide to Pro Football by Paul Zimmerman. (Well, the 1984 revised edition, so it's actually the NEW Thinking Man's Guide.) Anyway, it's just as good as I remembered, if not better. I first read it back in about 1988; then about 20 years ago I got the first version, from 1970, and read that one. It's also great, and he replaced at least half of it for the later edition, so they can almost count as different books.
There are other good football books, but none that seem to cover EVERYTHING so well. Despite the very broad scope of the book, it's loaded with specific details: strategy, anecdotes, character profiles. And it's very well-written, too. I really don't know how it could be any better, and I'm a naturally critical type.
Anyway, rereading it just made me want to give Zimmerman a little plug here.