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, 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.