[PP-1] Rewrite book module to use a standard, reusable API for creating hierarchies

The Book module is now in contrib.

So hate to be the negative person but with 15 years of small discussion is this something to still consider for contrib book ?

I think now that Book is in contrib, it's an excellent time to consider it!

But what use would this be for others? My understanding the original intent was to have a core API, using book as an example, to create this hierarchy.

So should this be closed and focus put into #344019: New implementation for database tree parsing ( taxonomy / comment ) → as this may be the closest thing in core as @catch mentioned?

That said I don't mind keeping open but don't know if I have the bandwidth to implement and rebuild book with such an API, and this has kinda been quiet over 15 years.

We can use the "Nested Set" approach and the Entity Hierarchy → module - it works great for large tree structures!