Don't request each book separately (multiple entities of same kind).

So if you need to retrieve 10, 30, 1000 or 10,000 books - do it in one request.

*. 10,000 probably could involve paging.

Suppose you had to retrieve Books, Authors and Publishers.

You can do 3 separate (parallel) requests: Books, Authors, Publishers - because each kind has its own endpoint.

But eventually, I needed to get all kind of mixes of entities, so I added also a generic endpoint - to retrieve entities by list of EntityId {string Kind, int Id}.

So I requested all the needed entities in one request: Books + Authors + Publishers.