I admit it. I love chess books. However I find them hard to follow. So I really like the Everyman Chess books. You can buy and download a book as an ebook, Kindle book, Chessbase format, and PGN file. This gives you options to view the material and have a board to move through the variations. The ability to go off on variations and jump back to the main line. Also you can check the evaluation of a line with a chess engine. Here is a link to Everyman's guide to the various formats.
There is just one problem. They don't actually sell the hard copy of the books any more. So I wanted a couple of books to work on an opening repertoire. I found them at Thriftbooks.com and Abebooks.com.