I have a friend who is starting to play bass. He wants to play for fun and doesn't want to pay for lessons. I offered to be his teacher for free. I'm not a professional teacher, but I do know plenty about music in general. However, I'd like to structure the lessons instead of just throwing random bits of information at him. Can anyone recommend a good book to follow? I'm looking for something that we can work through together, starting with the most basic material then slowly working up to the more advanced stuff.