Colaborative book writing
To sum up the comments: There is no such thing.

I also think that there is some reason behind this. Writing a book is quite some work and in fact, it is quite some more work than writing lecture notes. I guess one can put it this way: If the manuscript is written (in form as it would be written as lecture notes), then at least 80% of the work is left. Things that are left to do include:


Check if the material is organized in the best possible order, given the possible readers and lecturers a path how to access the material but also to offer additional routes than just going through the book cover to cover. 
Cross check all notation for consistency and clarity.
Build and, far more important, cross check the index. 
Build, if necessary, a list of notation and/or a glossary.
Recheck your list of references for completeness, but also if it still is up to date. Many textbooks also have a "commented list of reference" to give hint for further reading.
Edit, edit, edit, and edit again.


Besides all this things: There are many different writing styles, for textbooks even more than for scientific papers. If you have a bunch of authors and you did not fully agree on the style (by agreement or by chance), you will probably end up either with a never ending editing going in different directions or with a books that changes the narrative style several times and hence, is not a smooth read.

On the other hand, I, personally, think, that it is a good idea to write a book in a small team of two or three people. However, you probably need some joint writing experience to make this a success.