I'd recommend "The Dog Listener" by Jan Fennell. It isn't boxer-specific and it's not a training book in the sense of 'this-is-how-to-teach-your-dog-to-sit' sort of training, but it is an invaluable resource for establishing a correct relationship with your dog, with you clearly in the leadership position. From there, training your dog to behave in the ways that you want becomes a relatively simple task
It is reasonably priced and readily available. Just do a quick search online.