The Free Technology Academy is an organisation made up of several educational institutions from various countries that aims to contribute to a society that permits all users to study, participate and build upon existing knowledge without restrictions, software used in the Free Technology Academy virtual campus is free software built upon an open standards framework.
Linux from Scratch: A free open source ebook that will teach you how to build your own custom Linux distribution entirely from source code
WikiBooks: Wikibooks contains all kind of science and computing books available for free online, they can be edited but unlike the others, Wikibooks does not seem to have a PDF download option.
FreeTechBooks: A website full of free downloadable tech ebooks related to computer science, engineering and programming, text books and lecture notes, all of the ebooks have been legally released to the Internet community, check out the open source books in their FreeBSD section.
The Free Technology Academy is financially supported by the Life Long Learning programme (LLP) of the European Commission, you can download free Linux and open source related eBooks from their materials webpage, these books are released under the Creative Commons license, it is likely that new ebooks will be added to their course materials.
Some of the books available for download from the Free Technology Academy include “Introduction to free software“, “Gnu/Linux advanced administration“, “Gnu/Linux Basic“, “Introduction to software development” and many others.