One of the many defensive books in my youth football coaching library is Complete Guide to Installing the 44 Split Defense (Art & Science of Coaching)  I am not big on the 44 defense but this book has some very good general defensive coaching information, coverage explanations, communication, linebacker play and a pretty good chapter on the kicking game from a defensive perspective.

I recommend this book.  Its not a favorite of mine but a good addition to any coaching library.

Remember, Play for Fun and Winning is Funner.  

Coach Parker, Keller TX