Isn't books open source? I mean, you open them... and read them. It's not like it's encrypted or compiled...ah you mean the topic of the books are open source and they're freely available to download... silly me...
I downloaded and printed this little book. It reads quite difficult. Its not an encompassing book at all.. its just an installation manual for Ubuntu, I'd say rather download a book like Ubuntu Bible 1000pages + and is written and edited to make it an enjoyable experience, ive discovered these free books are a pain in the ass to read.
For the OpenSource followers on the windows (OpenSource exists on windows too you know), you can get started on programming for windows with SharpDevelop and C# books from here http://www.computer-books.us/. There are thousands of open source applications for windows. Check out www.codeplex.com and www.sourceforge.net .
Yeah, Lawrence Lessig has several of his books available under creative commons licenses. Available at http://lessig.org/. All of them are well worth reading if you are into online content ownership models and new media.
You didn't mention "In the Beginning was the Command Line" aka Cryptonomicon, by Neal Stephenson
OK, more of an essay really, but a very interesting read, it goes into the history of computers, software and how the free software philosophy came about.
Available here: http://www.cryptonomicon.com/beginning.html
I'm not sure if you're not liking the .pdf format, but if so you might like Calibre, the 'e book manager' which also has a built-in e-book reader. You can take a .pdf and change the format and read it with a nice e-reader screen.