FreeSoftware, and also Open Hardware needs free (as in freedom) documentation, otherwise they contradict the purpose of subject they are written about.You can visit the GNU project website for a more in depth analysis on Why Free Software needs Free Documentation
This is why I would like to present two less known books that are free:
Arduino starter kit manual: A complete beginner guide to the Arduino
Written by Earthshine Electronics (they sell electronics parts) is a rather complete book on the subject is easy format that can help even children and teens to get initiated to electronics without prior programming knowledge.
Simply Arduino (اردوينو ببساطة in Arabic), written by an Egyptian engineer, and probably the only book written about the subject in Arabic. It is available online for download with source code of the sample projects.
Quake1 portal gun
A portal inspired game using Quake1 engine based on: http://www.moddb.com/mods/quake-portal-gun
Video of the gameplay:
[p] - portal gun.
[q] - fire blue portal.
[e] - fire yellow portal.
[f] - grab item.
[w][s][a][d] - movement.
and it will copy all *.pak files into portal/id1 folder.just switch to that folder and run from command line your favourite quake engine (darkplace or ezQuake...)
$ cd openquartz/pak0/ $ make && make install
Hope you enjoy it!