The Ubuntu Pocket Guide by Keir Thomas can be purchased in dead tree form or downloaded for free as a pdf. Clicking on the title or cover pic will take you right to the page where you can get it. This page has the table of contents as well as a little more info. on the book. While I am not an Ubuntu user personally (Fedora is my preferred distro) I know there are a lot of them out there.
If you fill out the form at Tor.com they will send you an email a week, with a link to download a free e-book from one of their authors. You also become entered in a drawing to win an Asus eee PC Galaxy. Not bad. There will be, I think, 12 ebooks total. They come as PDFs with no DRM. Nice.
I’ll have a review of the first, “Mistborn” by Brandon Sanderson as soon as I finish it.