I am the defensive coordinator this season for the Keller Raiders Select youth football club in Keller Texas, and I am reading a ton of defensive books this season. Every month I buy one or two used football defensive books that are less than $10 on Amazon. I keep a few books by the TV, and when I get bored with a game like today's Cowboys game against the Vikings, I thumb through one of the books for some youth football coaching ideas.
One of the books that has been instrumental in my defensive practices, and one that I carry in my football backpack is AFCAs Defensive Football Drills by the American Football Coaches Association with a forward by Grant Teaff. For just $5 you really need to give up a six pack or latte and get a great defensive football coaching book.