I would think any book on dog training would apply to a 5-year-old. If anything it should be somewhat easier to train an adult because they are not as crazy as puppies. It was at about age 5 that Lightning finally seemed to grow up. He's 12 now, and I can still teach him anything. Since Stomps died a couple of weeks ago, I've been putting Lightning through his paces to mentally stimulate him. He still remembers things he learned years ago and has hardly been asked to do since (it helps when food is involved, but it doesn't have to be). Of course, you may to untrain your dog some things he's already learned, which I think is much harder. Good luck and let us know how it goes. And thank you for adopting a dog!