Hmm, good question. Sadly I’ve never really referred much to any books. My understanding of folds have just come from observation of everything I’ve seen. So that would be, artwork of others and fabric in real life.

This might sound kind of strange, but if you take a piece of paper and try making organic folds, or scrunch it, the paper holds form much better than fabric would. Take note of the creases, and the lighting. I think most of my understanding of folds comes relating it to paper. Naturally, when applying it to drawing actual fabric you’ll want to make it feel less stiff ;)

Even to myself this sounds somewhat absurd, but hey it floats my boat! Haha, let me know if it somewhat helps for you!