The number one reason for working in a bookstore is efficiency.

1. Remove the whole "get paid, walk to bookstore, hand over money" stage by just taking pay in form of books.
1a. This also reduces wear and tear on currency as it doesn't even change hands.
2. Don't even bother pricing the book if you're just going to take it home anyway.
3. "Browsing" time doubles as "working" time.