Here's an idea: make it just as easy to add one's accountant to the Zoho One version of Books as it is any of the other editions of Books.

This seems like a "duh" thing. Yes, I've read that I can contact support to accomplish this, but why should I have to do that? What would make Zoho think that an organization using Zoho One would be less likely to have their accountant require access to Books?