Marlowe asks her the same questions he asked the girl at Geiger's shop, but this time, the woman knows the answers. She catches on that Marlowe is asking a trick question because the book he wants doesn't exist.
She gives up the goods, and Marlowe gets a physical description of Geiger from the woman.
As Marlowe leaves the bookstore, it starts to rain. He opens the package he'd been holding. It's exactly what he thought it would be: pornography.
Geiger's racket is obvious by now. The guy loans out pornographic books from the back room of his store, fronting the whole operation as a rare bookstore.