I did a book in rst and liked that it had cool admonition, import, glossary, and index features that made it better than markdown for me. Still hate the heading conventions.
I have a custom pandoc filter for callouts and index entries. None of the simple lightweight markup languages has complete support for writing a real book. Writing custom rst code is a pain (and no one else in the world uses it). (I say this as a 25-year Python veteran and as a docutils committer!)