InternetArchive: The Internet Archive was founded to build an Internet library. Its purposes include offering permanent access for researchers, historians, scholars, people with disabilities, and the general public to historical collections that exist in digital format. This a great site to access many older works both in digital and audio versions. Free.
Project Gutenberg : Project Gutenberg offers over 49,000 free ebooks: choose among free epub books, free kindle books, download them or read them online.
OverDrive.com: OverDrive libraries add to their collections from a catalog of over 2 million eBooks, audiobooks, and videos. You can access OverDrive through the SCF Library database. It is also available as an App.