My own short answer is: they’re not.
I don’t know why, but I am often suspicious of people whose shelves are completely alphabetized or arranged by genre or size or, gah, color. My books tend to be more of a Pigpen-style cloud than anything organized, than anything resembling a navigable library shelf – maybe it’s a reaction to working in a bookstore for the last four years. People ask me how I find things, but I have a basically four bookshelves in my home – how far could they wander?
How do you have your books arranged? Defend your methods, please. Make me see the light.