Free textbooks available from sources such as CK12.org.  Click here to see a more comprehensive list.

Project Gutenberg offers over 33,000 free books for download.  The ebooks can be read on a laptop or on various mobile devices.  Project Gutenberg offers everything from classic novels to periodicals.

Check out Project Gutenberg here.

With FlexBooks you can build your own textbook.  This would be great to use with students who need to do some catching up or for students who like to be challenged.  

Build your own book here.

I came across another source for free online textbooks.  You can check it out here.