However, with graph databases I get the best - and even better - of both of these worlds. My book entity contains a pointer to the author(s), so to query the book and their authors is even faster than a relational database.
Many folks take this as a given, but the performance difference is often not that large, and may even be in the other direction, if you build the right index (e.g., on the AuthorID in this case). The index is essentially those pointers as well. If the data fits in memory (Neo4j, in the past at least, has pretty much required that), then the B+-tree index lookup is also very very fast. You would be surprised at how fast PostgreSQL can do even seemingly complex graph-natured queries, e.g., finding, for an author, the co-author with which it has written most books.
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Many folks take this as a given, but the performance difference is often not that large, and may even be in the other direction, if you build the right index (e.g., on the AuthorID in this case). The index is essentially those pointers as well. If the data fits in memory (Neo4j, in the past at least, has pretty much required that), then the B+-tree index lookup is also very very fast. You would be surprised at how fast PostgreSQL can do even seemingly complex graph-natured queries, e.g., finding, for an author, the co-author with which it has written most books.