You can grab my books/authors database, which is in the form of an SQL file called books_dump.sql. See the pg_dump documentation to learn how to create such file (section 24.1) and how to load such a file into your own database (section 24.1.1). I used the command "pg_dump --no-owner --no-privileges books > books_dump.sql" to create this file.
You can also grab this psql session transcript to see a bunch of sample SQL queries in action. It's good to step through this transcript with my database loaded up so you can try to recreate these queries, and play with variations.