Ajax Penumbra 1969 (Mr. Penumbra's 24-Hour Bookstore, #0.5) by Robin Sloan

August 1969 San Francisco. Ajax Penumbra seeks a book–the single surviving copy of the Techne Tycheon, a mysterious volume that has brought and lost great fortune for anyone who has owned it. Late one night, after another day of dispiriting dead ends, he stumbles across a 24-hour bookstore, and the possibilities before him expand exponentially.