I can see where that's needed at a certain point. I've been paring down my book collection over the last 20 years so I'd say it's close to half the size it used to be. But I know by memory where most of my books are. Other than fiction/nonfiction, I do put same author or same subject books together (I have a shelf of autobiographies, a shelf for Bill Bryson, one for Dave Barry, two for Tolkien and Sherlock Holmes, that kind of thing).