This story is great, but I had a little trouble understanding it. It could be made clearer.

Maybe this is my problem:

For example, our Person type below is able to load a “bookCount” column, no matter where it comes from.

Is this better?:

For example, our Person type below can optionally load a “bookCount” property that’s not a database column.

And later, you could say:

Now it’s natural to fetch persons and their book count without fetching all their books, using a simple join query:

Oh, and I suggest: Oh, and → And :)