CK12 is a great resource for students and teachers. Their flexbook site contains tons of online textbooks, all free, for teachers to use. You can also create your own flexbook on the site, combining parts of other textbooks from their site, or using materials of your own.
They currently have flexbooks in Physics, Math, and Biology, Science and Technology, but there are always more being created and added.
The site is relatively easy to use and I plan on creating my own Flexbook for my Physics class.
Many of them are in PDF files and you can download the Flexbooks app for iPad and Kindle. Many of the books have links and videos also. All of the books I've looked at are very well written and will be a great resource.
Why spend precious dollars on static textbooks when you can get some great ones online for free?