Online education books can be found at BookBoon.com. This site offers free and paid digital textbooks that can be downloaded in PDF format. The textbooks in this site are rated and reviewed with most of them receiving an average rating of 5 stars.
Books in this site fall under the categories of Business, Accounting, Career & Study advice, Economics & Finance, Engineering, IT & Programming, Languages, Marketing & Law, Natural Sciences, Statistics & Mathematics and Strategy & Management. Most of the eBooks that come under the category of Business are not free. However, they can be accessed during a 30-day free trial of the site's premium eReader.
The books are modest in size -- most run from 50 to 100 pages -- and use simple and direct graphics. All the books under a category can be downloaded in a single zip file. It is not necessary to create an account to download textbooks from this site. The company sells advertising that appears in the books. BookBoon says it limits advertising to 15 percent of the content, and so they're fairly non-intrusive.
Sites like Boundless, California Learning Resource Network, or CLRN, College Open Textbook Collaborative, Connexions, The Global Text Project, OpenStax, Open Education Group, Project Gutenberg, Saylor.org and University of Minnesota Open Academics also provide free online textbooks.