Wikipedia has a tool for assembling books of their best content. And that's a good idea!
We have a lot of high quality content here, certainly enough for a small book. You could write a book on Stack Overflow alone.
I'm not saying that it necessarily has to be integrated into the main site, but this is just a suggestion if anyone wants to implement it.
Stack Exchange probably could make money selling the physical books though. CC-BY-SA allows selling, as long as you allow others to make copies of your book, and credit the author.
What Wikipedia does is you can custom order any set of articles as a book (it even has a nice set of authors at the end, to satisfy the BY part of CC-BY-SA).
This could even tie into the docs feature.