Any books to help me learn about non-binary?

A close friend has "come out" as non-binary; I use quotations because somehow, upon them doing so, it just clicked perfectly. It made perfect sense. It took them a long time to figure this out and then share it, and I could not be happier for them.That said, beyond what they've told me about non-binary and what I've read about it on a few web articles, I don't really know much about it. I'd like to get a more thorough, perhaps less clinical understanding of what the non-binary experience is like. Part of the reason is that this is an area where I am not at all knowledgeable and I like learning about stuff, and also the fact that it is such a huge deal for my friend that I'd like to be more acquainted with it. Furthermore, they've asked that people try and refer to them as "they" instead of "she", but I fumble this particular ball at times and I think it's because although I care deeply for my friend and want to support them any way I can, I have not perhaps entirely internalised the concept.Hence I was hoping there might be a book out there that might help me understand more about the experience of living non-binary in a very binary sort of world. Unfortunately, the hits I get when doing searches on Amazon miss the mark terribly.Do you have any recommendations?