While Creative Commons licenses are the most frequently used for e-books, other licenses can be used to allow for the free reading of books. Noteworthy among these is the GNU Free Documentation License (FDL), created by the Free Software Foundation to allow software documentation, manuals and other text to be distributed with strong "copyleft" provisions compatible with the GPL software they're meant to accompany. The GNU FDL can easily be applied to e-books; many e-books have been released with this license and with other Free Software Foundation licenses.
The idea of copyleft is that licenses can be used to prevent someone from taking from the commons without also giving back. For example, when a book publisher adds commentary and illustrations to the text of a Shakespeare play, the resulting book is covered under copyright and permission must be given for redistribution even though the underlying work is in the public domain. This would not be allowed by a copyleft license. The Creative Commons SA licenses have weak copyleft; the GNU FDL is stronger, and even forbids the use of DRM. It's not clear whether it would be legal to distribute a GNU FDL e-book to a Kindle e-reading device without permission from the author.