Cambridge University Press is perfectly willing to publish books which are also freely available on the web (at least in 99%-complete draft form). I'm not sure how far they'd go in terms of the most liberal Creative Commons license, but here are a couple of examples of it occurring:

http://theory.stanford.edu/~tim/ -- scroll down to find the link to "The AGT Book". http://www.cs.princeton.edu/theory/complexity/

I think there are additional other examples from Cambridge; those are just two I knew off the top of my head.

Edited to add: Sorry, I didn't notice that David Speyer already pointed out Cambridge University Press in the context of Allen Hatcher's Algebraic Topology book.