Really free chemistry books #2: @bookboon

About a year ago I wrote about free chemistry books. No, not illegal copied books, but really free books (though, not necessarily Open). Actually, the books I discussed last year, those are out of copyright, as they are old. But, just today I ran into an advertisement for free books by bookboon.com (twitter:bookboon), and these are new books. I have not browsed them yet, but you can download textbook PDFs from these sections: biology, chemistry, chemical engineering, math, nanotechnology, and several more. I do not know about the quality of these books yet, and they author names do not immediately ring a bell. Does anyone have experience with these books?