Kur rasti nemokamus ir gerus universitetinio lygio vadovėlius anglų kalba?

"The University of Minnesota’s Open Textbook Library has more than 160 titles, and OpenStax College, a foundation-funded nonprofit affiliated with Rice University, offers a dozen titles, mainly focused on widely offered introductory courses, with more in production."

(Informacija iš:  http://www.nytimes.com/2015/02/26/your-money/putting-a-dent-in-college-costs-with-open-source-textbooks.html?ref=your-money&_r=0)