The are a few ‘free’ ebook organizations around providing a range of models. Bookboon has an ads model which appears to working. The quality looks OK. The main limitation is students have to go to the site to get the book, you’re not allowed to load it onto your site. That seems more than fair.
Here’s a bit about them
“Bookboon.com publishes free and openly available eBooks for students and business professionals. The books can be downloaded in PDF without registration. Our mission is that students should be able to go through university without having to pay for textbooks.
How we make this possible – Your future employer pays for your textbooks
We finance our textbooks with a low number of high quality advertisements. The advertisements are mostly employer branding from companies who intend to recruit the students reading the textbook after they graduate. So in essence, your future employer is paying for your textbook!
We have set a 15% advertising limit per book. This limit has been set together with professors, business professionals, and student organizations in order to ensure the whole experience is in line with the high quality of our books.”