.epub is the best Project Gutenberg format for most books. The wonderful Windows program Calibre has third-party add-ons that will remove Kindle drm and allow books to be converted to .epub format. FBreader works with.epub, but other readers such as Dorian are prettier but lack some of its functionality, so it's a good idea to try several readers and see which you prefer.

You can get many audiobooks for free from libraries, and the MP3 versions can often be copied directly over to a player and kept permanently.