Feature Request: Book Showcase

[RobHarveyDev]

Idea
A place on a users profile page to show off favourite books, similar to the games showcase on Steam.
This would be different than just a book favourite system as there'll be a limit (maybe 5) of books you can showcase to keep it presentable on the profile without a huge scrolling list.

Implementation
I thought it could be a shelf similar to "Read", "to Read" and "Reading", but I'm not sure on how exactly that would work and how it could be affected by the PR to refactor those in #2170

I'm not sure where to start on a potential PR so any advice/ideas would be helpful.

Here's the steam showcase for reference: