Many authors nowadays release books under free licenses, such as the GNU Free Documentation License (FDL), Creative Commons License (CC), and even public domain. An advantage of the FDL is that you (the reader) can use the text of a book in any way conforming to the license; for instance, you can create a new, perhaps shorter or longer, book by writing some of your own text and also by cutting and pasting from other FDL books. Of course, if you ever publish such a compilation, - even merely online, - you will have to release it under FDL, specify that it is a compilation, specify the sources, publish the modified sources, and so on, according to the terms of the FDL.
At this time (2010), free e-books are available on every area of university-level mathematics and physics. Not all of these books are released under FDL. Here are some of the science e-books released under FDL that I could find. (The files are attached below.) I also note the URL from which these files were downloaded; it is quite possible that updated versions of these books will eventually appear.
Mathematics
Physics