I really love the idea of the dart book series and hope in the future other books could be split into a few volumes if the authors are afraid of thousands of page books. Because I have read other books from kodeco - raywenderlich such as iOS apprentice or swift apprentice. At the beginning, they are pretty good, with great explanation but towards the middle or half way through the books, then most of the code are not even explained since the aim of kodeco is to help people learn to code, and beginners in particular. so I hope the authors could squeeze in as many details and explanations as possible. If one volume is too big to fit them all, please split them into several volumes. The reason why we buy books is because there is something which is not available in the documentation. Think of a book as a mentor or teacher, who the newbies have to completely rely on. so hopefully in the future, we could see more quality books from kodeco.
BTW, personal opinion, I think it would be best that only two authors work on a single book at most because more authors work on the books might compromise the quality as one might not know what the previous authors did - probably they made some assumptions that concepts they address might have been well explained so they did not even bother to give more details. In other words, there might not be a consistence of connection between the previous author and the author who works on the current section.