Because he wanted to test Agnes Louzier who was supervising the book store in the absence of Geiger.
We know later that the book shop was not really a book shop but a pornographic place in a disguise.
Philip Marlowe enters the book shop and then the lady asks in a gracious hospitality voice:
“Can I be of any assistance?”
Which is very uncommon for a clerk who maintains a book shop. Her appearance is also odd for a book shop clerk. She was dressed in such a manner as if she was trying to impress customers.
Then Philip asks for the books “Ben Hur 1860 (III Edition) one with the erratum on page 116” and she first asks “First edition?” later for “Chevalier Audubon 1840.”
“I am afraid not and not at the moment…”
…respectively for his queries.
Then Philip — in a bit of a surprised and sarcastic voice — says
“You do sell books? Hmmm? ”
This implies that he had known that she knew nothing about books and the place is not a rare book store. This is later confirmed when a person walks into the store and walks straight into the back room.
And — actually — those books are non-existent books. No books with such names exist. Philip made up those names to test her.
This is confirmed when Philip Marlowe walks into other bookstore Acme Bookstore and asks for the same books. The proprietress (Dorothy Malone) there says nobody knows where the books are available because no such books exist.