Books, I will have to dig out my info....I actually did a study on it, and in fact, included the info in an article on trousers. While Levi's themselves are nothing new (going back to approx. 1849-50 or so), the belt loops themselves were not a part of the jeans until approx. WWI or so....
Once again, I don't have all of the info in front of me, but my last research says this. Someone else, however, may have different info.......