I agree that the coherency of the text book would be poor if everything was written by different authors. In fact, it is often so jarring when I read a journal article with different writing styles, that it is one of the easiest ways for me to identify plagiarism in student work or when reviewing for journals.
I guess the answer would be to have particularly committed and experienced editors like you mentioned in your attempt.
Perhaps if the book started of as quite a basic book, with key concepts, written by one person. Then, over time, that person (or small group of people) could accept pull requests from the community, and modify them to fit the style of the original book.
This could work quite similarly to most open source software projects:
1) Start of with one or a few people
2) Build only enough functionality for a working/useful piece of software
3) Over time, build up functionality via community contributions (and further contributions from the original contributors)
4) The direction of the project is steered by a group of people with its best interest in heart
Note how in the example of software, there is a fully working thing (even if it doesn't have all the bells and whistles attached) quite early in the process. Also, the overall vision for the project is kept in line by the original maintainer, who runs an eye over each pull requests and is free to make suggestions for changes. If there is any major disagreements, people from the community are free to fork the software and make it suit their needs.
I don't know how practical this is, in that a text book without huge amounts of detail may have less usefullness than a piece of software without huge amounts of functionality. Perhaps there are some fields which would benefit from such an approach, while others really do need to have a comprehensive, "finished" book to be of any use to anyone.