Well, there are many diagonals for the Bishops and Bishops do move diagonally, so saying "I like to utilize my bishops on the diagonal" doesn't make a whole lot of sense.
That said, if you are defensive minded, and don't want your Bishops hemmed in behind the pawns, possibly the Caro-Kann is the line you should be looking at. Maybe pick up "The Caro Kann: Move by Move" by Lakdawala. That said, you still have to look out for opportunities that White gives you and know when to attack!
I am a returning player after a 20 year absence to the game and want to learn a defence to e4 other than the standard e5. I am slightly defensive minded and like to utilise my bishops on the diagonal. My preferred opening on white is the English, however I am put off the sicilian due to the amount of theory associated with it. Any recommendations for a defence and book?