SOSTX

As you might already know, California has embarked upon getting out from underneath the high cost of text books.  And, what's nice, they are providing the works for free (offered under a Creative Commons  license) and they are available for download as PDF's via CK12. No sign up required ... just click to download. 

CK12 offers downloads for Chemistry, Calculus, Trigonometry, Geometry, Biology, Life Science and Earth Science.

http://about.ck12.org/

CK12, however, does not plan to stop with PDF's.  It's working on a social network and community (web based).
They currently have a web based search for textbooks which, if it doesn't find CK12 materials, provides a link to found "Chapters" and "Web" (which are links to WikiPedia).  For example, searching for "history", one can find a WikiPedia reference for the History of China and open it in a popup window which allows for browsing the content

Following their "Chapters" link, one can choose an "action" on a resource in the listing.  Options include "add to flexbook" as well as view and edit.  Adding to flexbook one can edit - Title, Descripition, Categories, Standards, Parameters, and Tags ... on line.

One can also create a Flexbook .. from scratch adding content and even customizing via their tool called the "Flex Assembler".
Take a gander:
http://flexbooks-wiki.ck12.org/index.php/Introducing_the_Flex_Assembler

For more information:
http://flexbooks-wiki.ck12.org/index.php/Main_Page