Response Mapping for Join Queries?

If I have a resolver that is an SQL query (on an RDS database via the Data API) that has a join on it, how do I map the response to the query to the 2 types in the schema?

type Book {
  author: Author!
  title: String!
  etc
}

type Author {
  name: String
  etc
}

and the resolver SQL query looks like

select * from books left outer join authors on (book.author_id = authors.id

It seems like such a common thing to do I'm sure I'm missing something obvious but also I'm surprised there isn't an example in the documentation.