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