I was pleased that this site appears to be legitimate, i.e. not providing illegal downloads of copyrighted textbooks. I tried half-a-dozen of the textbooks. All seem to have sites where the copyright-holder allows free downloads. So it is probably OK for BookBoon to allow free downloads from its site.
BookBoon does not seem to offer any open-licensed textbooks. Open textbooks have major advantages over other free textbooks. It is legal for instructors to incorporate open textbooks into their Learning Management Systems and place the textbook on their own websites. Unless derivatives are forbidden, the instructor can modify the textbook to better fit the students and the class. These steps are not possible with a copyright-all-rights-reserved textbook like the ones on BookBoon.