I have kept the books in my library in a database for many years, but as the database has migrated from one Palm application to another, and now to an iPad app, the database's capabilities have grown. Sometimes some data has dropped off, or numbering has been lost, so I have been trying to work through a shelf each weekend checking the database against the shelf's contents to make sure that all the books are in the database, each book has its library number pencilled lightly inside the front cover, and most recently, that each record in the database includes a photo of the book.
Today I was in the world history stack (stack IV), shelf 4 – the shelf devoted mostly to ancient Greece. The sculptures of the Acropolis and Pheidias' Athena Parthenos should give it away. It sits above Byzantine history (which features the icon of Christ Pantocrator and you can barely see the minarets of Hagia Sophia), and below Roman/Italian, with tiny models of Saint Peter's and the Leaning Tower of Pisa. I did leave three books unentered at right as the program started crashing. I'll add them later.
But this helps me tell when I'm in a bookstore whether I have a certain book already or not – or whether the copy that I have might be an older one. I only note location as room in the database (here "study"), and not by stack and shelf, because that would be compulsive-retentive.