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

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.

I came across this post and wanted to share it.  As classroom teachers, we struggle with acquiring up-to-date textbooks.  This post shares a site where free textbooks can be viewed and some even downloaded for classroom use.  Check it out here.