i'm so surprised at all the hundreds of free downloadable math and physics textbooks at freescience (e.g. http://www.freescience.info/books.php?id=4&PHPSESSID=c3046af36f33033bc738fa34f64005be , covering almost every university level and even graduate level topic, with professional typesetting and diagrams, table of contents, etc... some textbooks have over 500 pages. i can't find these titles at amazon so apparently these books (most of which are only about 5 to 10years old) have not been published. so what gives? is there a reason that they are offered for free? seems to good to be true. some authors don't have have phd titles (but most do) but many are college instructors, so their credential seems ok. should i start reading from these books and trust their content?