You may know that Wikipedia has many open source books online (i.e., entire books available for free) here.
There's a nice section on education (here), and take note of:
Teachers' Toolbox is an open source Wiki book on teaching methods and ways to improve teaching. The toolbox is intended to give you an overview of methods you can use when teaching in general. Hopefully the toolbox can become a discussion forum for teachers, by using the discussion pages in the wiki system. We hope it will help you to find new ways to activate students and improve their learning. It's not intended to be a long theoretical analysis of pedagogical methods, rather emphazises brief introductions to different ways to teach. Hopefully people will contribute with case stories and examples of teaching material you can develop further.