Thank you @amaster507, that did it!
I reset my dummy database and reloaded the data, this time with the data below (last line added):
_:Book.123 <dgraph.type> "Book" .
_:Book.123 <Book.id> "Book.123"^^<xs:string> .
_:BookTitle.123 <dgraph.type> "BookTitle" .
_:BookTitle.123 <BookTitle.id> "BookTitle.123"^^<xs:string> .
_:BookTitle.123 <BookTitle.title> "War and Peace"^^<xs:string> .
_:BookTitle.123 <BookTitle.book> _:Book.123 .
_:Book.123 <Book.bookTitles> _:BookTitle.123 .
and now the results of the same query look like this:
{
"data": {
"getBook": {
"id": "Book.123",
"bookTitles": [
{"title": "War and Peace"}
]
}
}
}
I really appreciate the response.