Free E-Books with Advertisements

Today a colleague brought to my attention Bookboon , a web site that offers free e-books.  They currently offer textbooks, business books, and travel guides.  The only "catch" is that each e-book has advertisements in the text.  Perhaps this business model will work and expand.  After all haven't we watched television shows with commericals for two generations?  If it can save students money, it should work.