Bookboon is a service that offers free etextbooks to high school and college students. The textbook section of Bookboon offers more than 500 digital textbooks. On Bookboon there are etextbooks available in ten core subject areas with additional subtopics with each subject area.

Source: http://www.freetech4teachers.com/2016/05/a-large-collection-of-free-etextbooks.html