Bookboon is a free service offering free full-length textbooks, travel guides, and business books in digital form. The textbook section of Bookboon offers more than 500 digital textbooks. On Bookboon there are etextbooks available for twenty-five subjects, but the bulk of the etextbooks are focused on Economics, Engineering, and IT. You can browse the title lists to find a book you want or you can search Bookboon by keyword. Bookboon hosts books written in five languages. All of the books are free to download. The only catch is that you have to provide an email address before you can download the books.
Applications for Education
Bookboon's books are targeted to university students, but that doesn't mean that some of the books couldn't be used with high school students. And since the books are free it wouldn't hurt to download one that you think might work for your class and use excerpts of it to supplement other materials that you are already using in your classroom.