In fairness, if this was in a relational data store, the same code as above would probably look more like...

SELECT DISTINCT authors.some_field FROM books JOIN authors ON books.author_id = authors.author_id WHERE books.pageCount > 1000

And if you wanted to grab the entire authors record (like the code does) you'd probably need some more complexity in there:

SELECT * FROM authors WHERE author_id IN ( SELECT DISTINCT authors.author_id FROM books JOIN authors ON books.author_id = authors.author_id WHERE books.pageCount > 1000 )