"Say I wanted to read a random sci-fi book by an author who isn't partcularly popular...what are the chances that I'd be able to download it? Just wondering! "
If you are interested in a Kindle, the best thing to do is to go Amazon and look up a bunch of books. See if they are available in Kindle format. It's hard for me to say "Yes, you would be able to find that obscure author or No, you would not be able to find that obscure author". Go look and see for yourself.
Art books would totally not work on a Kindle. The Kindle is only black and white. The best books for the Kindle are novels, where there are no photos or other artwork.
It's really great to be able to read the newspaper, even while smushed in the middle seat of an airplane. That's impossible with a full sized newspaper, no problem with a Kindle.