The GPL was an incredibly significant stake in the ground. Far and away his greatest contribution. That and the audacious idea of building a complete free operating system. I think he overstates the role of the FSF and the GNU project in putting the actual OS together. In the end, there's probably more from BSD and Sun and other Unix vendors than from the FSF. Unix itself was really built by a collaborative community, and when you look at a typical Linux distro, it's got stuff from many hundreds of contributors...but so does a Unix distro like Solaris!
But the *idea* that the community shouldn't just contribute their software to a framework that was owned by a vendor, but should supply all the pieces that were necessary for a complete OS, that was Richard's, as far as I know (though some of the early BSD guys may have had similar thoughts.)
Back to the book itself...I was really impressed. I'm sure that Sam didn't get every detail right (though lots of people read the book and commented, and we'll keep refining it through reader comments online), but he seemed to me to capture Richard in a way that I've not seen before. Let me put it this way. Richard and I have butted heads on more than one occasion. But this book gave me more respect for Richard and understanding of where he's coming from than anything else I've read.