TITLE: Polymath-type projects for textbooks?
QUESTION [14 upvotes]: Why aren't there any "crowdsourced" projects for mathematical textbooks?

Every year, many mathematicians put a lot of effort into crafting their own lecture notes or writing textbooks (or also research monographs). Why aren't there any open-source "crowdsourced" cooperative efforts towards book writing? The pros of this kind of collaboration are rather obvious.

REPLY [12 votes]: The Homotopy Type Theory book is massively collaborative and regularly updated (through version control). It's also the definitive textbook in its field, and I think Voevodsky himself contributed to it. In particular, it is mentioned that:

We have released the book under a permissive Creative Commons licence
  which allows everyone to participate and improve it.

I've found it to be particularly well-written and comprehensible.