Since this is about timelines, I think it's important to keep another timeline in mind - the one leading towards completion.
I know - and totally get - the argument that it's tough to know when to quit gathering data and info and pictures and documents, and declare that part done, now on to the writing, but as Ric just said, this book is much more narrowly focused and so deciding when "enough" is in fact enough should be much more straightforward. We also have to be aware that this is a group effort, with a lot of people coming forward with their own time, money, and research findings, in expectation that the finished project they are working towards will be delivered in a timely manner.
And that's all I've got to say about that.
LTM,
Monty Fowler, TIGHAR No. 2189 EC