NFL Rewind is something I find to be invaluable. I can't think of any books right now. I learned a lot when I had the opportunity to intern as a coach for a high school team. And from there, I try to just build my knowledge any way I can. I'll say that Matt Bowen, who is a former NFL S, writes articles that I find to be very useful and informative. You can definitely learn a lot reading what he writes.
But watching coach's film on the NFL Rewind teaches you a lot just from observing how things work.
Thnx man, I'll definitely check out Matt Bowen!