Ok, so we all have our own particular neuroses, but I'm convinced that one of mine is not that weird. It's the simple fact that I prefer to shelve my books by size. All the hardcovers together, all the trade paperback together, all the mass market together, etc. I can't explain why I do this, other than the fact that it not only "looks pretty", but it gives me a great sense of contentment (this may have been born of years of not having adequate bookcase storage, and being forced to do things by size because it makes stuff stack better).
So, how do other people shelve their books?