That’s wonderful to hear!
Of course, we’d love to continue to improve the organizational features on Observable, to make organizing and finding your notebooks easier — this includes better search as well as more advanced collections features.
But let’s talk about what can be accomplished today.
For what it’s worth, collections are non-exclusive, and many-to-many. So you can have a notebook that is at once in your “Mapping”, “Needs more editing”, and “D3 Techniques” collections, each of which may be public or private — focused on presenting the notebooks to others, or on organizing it for yourself.
So you could, if you wished, create collections for each chapter of the “Math book”, as well as a single ordered collection for the entire book, and have your notebooks exist in both, if you liked.
Observable notebooks, in their wiki-like nature, are also themselves useful for organizing. For sub-chapters, or to-do lists, I often mind myself putting a bulleted list at the top of a notebook with links to other related notebooks inline.