You can find many open resources on the mega sites listed below. Please note that varying degrees of "open" exist between, and even within these sites. Remember to assess each individual resource for its appropriateness, accessibility, and level of openness.
The links below are open resources mega sites that offer books as well as other OER teaching tools.
"Open Education Resources (OERs)" by TRU Libraries is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
Based on a work at http://open.bccampus.ca/bcoer-librarians/bcoer-guides/.