Books, that is. Believe it or not, some dude has blogged about how to maintain your bookshelves in the right way.
Personally, I go for the two-layer deep, cram any more in any gaps you find, and hope the damn thing doesn't collapse approach.
And I have a dispute with: RULE #3: COROLLARY B -- If a book is allowed to be "borrowed," a replacement is purchased online as soon as the original leaves the house. No attempt is made to recover the "borrowed" book.
Yeah, lend someone a book. Good one. You'll have to fight for them first.