I get terribly frustrated by how bookstores choose to shelve books. You find pure non-fiction science books adjacent to fantasy novels. Books by Isaac Asimov, probably the author who has the widest distribution of books across categories, almost always end up in Science Fiction/Fantasy. Mysteries, biography, science textbooks, all in the SF&F section.
Sounds like a simple question, but as many booksellers will tell you, it's not. Does that biography with lots of travel stories go in biography or travel . . . or is there space to put a copy in each section? What about books by or about Hillary Clinton and Rudolph Giuliani--are they placed in politics, history, or biography . . . or somewhere else? Or that great, lost literary mystery that has cross-appeal? Importantly, where do you think your customers will expect to find the book?
When you add it all up, booksellers are faced with an amazing number of possible shelving permutations each day and divvying it all up in a finite space--sometimes very finite--is a challenge worthy of any statistician.
The key, booksellers noted, is to know your inventory and your customers--and to think outside the box."
--David Grogan, "Take This Book and Shelve It: Confronting the Category Ghetto," in Bookselling This Week