This proposal for a high-holy-days book, possibly billed as a machzor companion, suggests the following types of content:
Q&As related to particular parts of the Rosh Hashana and Yom Kippur machzors.
Q&As related to Halacha and How-tos of Rosh Hashana and Yom Kippur.
Q&As related to ideas about Rosh Hashana and Yom Kippur in general, and possibly Teshuva in general.
Our first step, before we can start to prepare material for publication, is to collect links to candidate questions. Please use the Community Wiki answer here to do this. We are looking for interesting questions with good answers, especially those featuring our own writing. See our past publications for examples. While you're looking, please feel free to improve the things you find on the site if you can. And if you have a question and don't find it, please ask it (so if it gets good answers it can end up in the book).
Here are some relevant tags (thanks Isaac).
The categories suggested here are just starting points. If you think of another category that we should be looking at, edit it into the answer! At this point we're gathering input to later winnow down into a publication; if you think something is worth including, please link it.
As of Sunday, July 5th, we have enough links here to make a respectable-sized book. Feel free to continue adding links if you find questions that would be worth including.
Call for edited submissions: Part 1: Machzor, Rosh Hashana, Part 2: Machzor, Yom Kippur, Part 3, halacha and how-tos, Part 4: themes, Part 5: Selichot. When you edit a question for submission strike it out on the list in the answer here. (Don't delete!)