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.