The Feynman Lectures on Physics Now Completely Online (2014)
openculture.com
openculture.com
https://archive.org/details/TheFeynmanLecturesOnPhysicsVolum...
You can download them in several formats, including PDF and even pure ASCII.
I know the Archive doesn't have the strictest policy on copyright (tons of old commercials, magazine scans, and other ephemera that most people see as a grey area is available), but I doubt even their views are lax enough to look at this as kosher.
While the original term of 14 years may have struck a good balance between motivating creators and moving their creations into the public domain, copyright laws have gone mad in the past century.
I'm sure if you try, you can find a more recent book. This is just some more food for thought. What length of time do you believe strikes the optimal balance between incentivizing creation and adding to the public domain?
The argument can hardly be made that this applies to Feynman. And in this case -- especially with such a treasure trove of information -- I cannot really see much of a purpose in it not being free to distribute.
For math there's The Princeton Companion to Mathematics [2]
[1] http://www.amazon.com/Computer-Programming-Volumes-1-4A-Boxe...