Hundredzeros.com is a great place to look for new books to read. A lot of the books are small, self published offerings, but sometimes you can the big publisher’s books as well. I’ve found useful books of each type, depending on the topics.
They have them separated out in various categories (scroll to the bottom). It’s great to download a few before heading out on a trip! Prices do change, so be sure to double-check the price when checking out with them.