At Bookboon.com You can get over 1.000 e-books for free as pdf-dokuments. An individual registration is necessary.

Doab - directory of open access books
The directory of open access books offers the access to a collection of 1100 open-access peer-reviewed e-books.

Google Books
Google books offer a large variety of e-books. In a number of cases there are only parts of the e-book available.

HathiTrust is a partnership of academic & research institutions, offering a collection of millions of titles digitized from libraries around the world.