Mainstream Sources
The sources mentioned below may offer content that is free or available for sale. Digital electronic text and digital audio content may also be provided to patrons through public libraries. Check with your local library about the resources listed here, as well as other services to which your library may subscribe. Online book stores such as 
Amazon.com and Powell's Books also sell audio and electronic books.
Audio
Digital
- NLS Fact Sheet Selected Sources for Electronic Texts
- Alex Catalogue of Electronic Texts
- Bartleby.com: Great Books Online
- Berkeley Digital Library
- Bibliomania—Free Online Literature and Study Guides
- BiblioVault
- Books24x7—California Digital Library—Repositories
- Digital Book Index
- eBook Locator
- eBooks.com
- ebrary
- Electronic Text Center at the University of Virginia Library
- eReader.com
- Fictionwise, Franklin Electronic Publishers
- The Internet Archive
- Internet Public Library
- Microsoft Reader Catalog of eBooks
- Mobipocket.com
- The National Academies of Science Press
- NetLibrary
- The Online Books Page
- O'Reilly's Safari Bookshelf
- Project Gutenberg
- Safari Books Online
- SafariX