I'm under contract for a non-fiction project that requires lots of research, reading dozens and dozens of books from the library, downloading half the pages of the internet. While I like research, I'm itching to start creating the chapters I have in my head, especially the first.
How do you balance research and writing? Do you fill up ten notebooks with sources and quotes and information before you write word one? Or just dive in and correct your mistakes as you find them?
How extensive or specific is your brief eg. how much of a preliminary outline has been provided by the client, beyond the basics? Or none?
You're asking when does one know when to cease research and then begin writing? In many ways, as I have found in some other areas of work, the cut-off point almost determines itself.
But having said this, the point is underscored by a number of questions:
if this is your first such project or not, the extent of the brief, the type of book, your level of curosity and whether you are detail-oriented or not, if the book requires this or not, the audience/market and your assumptions about it, your deadline, the type of research you're undertaking (primary and/or secondary),and corresponding fact-checking, among others.
As I understand that you're greatly involved in music, for the exercise I'll assume that the book is either for a young general reader ('Generation Y'), or the music fan /follower market. The characteristics of your reader here will have a relationship with the above-mentioned points. These books generally don't require the sorts of research that a serious careerist biographer will undertake.
In relation to research procedure per se, speaking strictly for myself, I try to be as deep, broad and as methodical as I can be. I often have to undertake both primary and secondary research, and I will write more notes and download more articles than I will ever use with the piece. This is to obtain what I call 'mastery' of the material. With reference to my brief, I intuitively know pretty much when this point has been reached. As always, your writing should clearly reflect / illustrate this.