http://knol.pwNOTE: The sign in uses Mozilla Persona. To create a Mozilla Persona account, just enter an email address into the sign in area and it will prompt you to create an account. And yes, it takes Bitcoin.
EDIT: I want to emphasize how impressed I am with the selection. A user on filesharingtalk.com said it was the best ebook site on the net.
If this benefits you, please consider giving me a tip. With a 95% discount, even the lowest amounts help.
19w83D6kJpaLPPRL6EKqNgAQtBpeQxWMuZ
Another good resource is
http://libgen.org 100% free, and it also has a database of scientific studies. The only issue is that the downloads seem to stop short. This can be easily remedied by using a download manager that has a resume function.