I recently came across the online textbook resource Boundless which has over 20 different online textbooks from various subjects. Here are some of my other favorite online textbook resources.
HippoCampus.org Free education resources for middle school, high school and even college students. Hippocampus has course materials in Math, Science, Social Studies and Humanities. Many of the textbooks even have video resources.
USHistory.org has free online textbooks for high school students and teachers. There are three textbooks, US History, American Government and Ancient Civilizations.
Open Culture has around 170 free textbooks in a wide range of topics and for various levels. You just need to go explore these resources.
Two other great resources although not really textbooks would be Crash Courses YouTube channels for US History and World History.