Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
LibriVox - free public domain audiobooks
Project Gutenberg - Project Gutenberg offers free ebooks to download.
Audible - Audible is the world’s largest provider of premium audiobooks and spoken word content.
Apple Books - The all-new Apple Books has been redesigned to make finding, reading, and listening to books a beautiful, effortless experience on iPhone and iPad.
Mobilism.org - A home for book lovers, app enthusiasts, and much more.