Free as in Freedom is Richard M. Stallman's biography by Sam Williams that is available as an Open Book by O'Reilly.
It took me quite some time to fully comprehend the meaning of the word "Free" in the definition of "Free Software". But when I did finally understand it, its enormity was astounding. And the reason why Richard Matthew Stallman (RMS in short) tenaciously sticks to that word (and insists that others should too) when alternate definitions such as "open source" have become more recognizable (or fashionable) is interesting reading. Free as in Freedom is RMS' biography by Sam Williams and is a fascinating story of the role played by others in the FOSS movement (Eric S. Raymond, Bruce Perens, Linus Torvalds and a number of others from the FOSS world), software technology, early hacker culture and gives a glimpse of what we see today in the world of hardware, software and Internet.
His work on Emacs, GCC, GDB, gmake is highly respected and the GNU Project forms substantial component of the GNU/Linux operating system. In fact, GNU Project had all components of the GNU Hurd operating system which it set out to develop, except the kernel.
Sam Williams builds a compelling narrative in his attempt to understand RMS and find what motivates him in his quest for freedom. RMS comes across as a mix of genius, obstinate (sometimes even abrasive) but no one, not even his detractors, deny the quality of his body of work (GNU Project) and the strength of the GNU GPL. The biography is completely non-adulative of its subject, in fact, at times a little critical of the subject. But a compulsive reading. Everyone interested in FOSS must read this biography.
Once you have read Free as in Freedom, you will find other open books from O'Reilly entitled Open Sources: Voices from the Open Source Revolution. Website of the Free Software Foundation discuss software licenses and the choice of the words "Free" and "Open Source".
If you recollect, on Steve Jobs' passing away, RMS said in his blog "Steve Jobs, the pioneer of the computer as a jail made cool, designed to sever fools from their freedom, has died." He also followed it up later to clarify that he is not glad that he is dead, but glad he is gone (But even more interesting is his follow up on this post, correcting his quote for the purpose of "accuracy." Typical of RMS, his penchant for accuracy!) There are any number of responses to this, ranging from incisive to lunatic. That is exactly the kind of response RMS has received all his life. And that is exactly what RMS is. You may love him or hate him, but no way can you ignore him.
World does need geniuses like RMS to help "keep its head on its shoulders" while it dreams of "walking with its head in the clouds."