I have two "barbecue" cookbooks on the bookshelf in my office that I refer to often. Both of them have been suggested in this thread...
Adam Perry Lang's Serious Barbecue
Andy Husband and Chris Hart's Wicked Good Barbecue
I should note that I own almost every barbecue book that is available (meaning in print now, and not self-published), and we have just about every book suggested here for sale in our store, but these are the two I love. Adam Perry Lang has a newer book called Charred and Scruffed that I really like as well.