[hailpixel]

Honestly, whatever suite of tools allows you to organize your research, thoughts, and easily write text. Remove as much friction as possible. When we wrote The Workshop Survival Guide (https://www.workshopsurvival.com/), we used post-it notes on a wall and google docs. It allowed for a fluid collaboration.

On a more meta note, how you approach writing your book is probably more impactful than your toolchain. If you're working on a book that educates (rather than, say, entertains), I'd like to share the writing framework that my longtime collaborator, robfitz, describes in Write Useful Books (https://www.usefulbooks.com/). Basically, treat your book like a product, find your audience, and test your subject matter with them as your craft your prose.

* Disclaimer: Rob and I run Useful Books, a community and toolset for non-fiction authors.