Excelent idea! My opinion is, though, that anything under 25-30 is irrelevant, so 25 would be very apprentice level.
For example reading 10-15 books in extremely diverse and varied genres, like scifi or fantasy, with their maybe hundreds of very different sub-genres, definitely does not make one experienced or specialist. Maybe in non-fiction - reading 10 books about Python probably does the trick? Not in genre literature, though.

This is something we're planning to work into discussions. When readers see discussions it's helpful to understand who they're talking with. Badges are a good way to quickly showcase that readers highlights.

This idea could also allow users to build a reading reputation similar to Reddit karma that book groups could use to vet new members.