What's in a title?

Have you ever looked at the titles on a library shelf and wondered why so many of them echo other books? Patricia Cohen in the N.Y. Times speculates about titles that have led to a series of take-offs. Freakonomics is one that started other authors and publishers thinking about similar titles. This is not a new phenomena, as Cohen points out Gibbon's Decline and Fall of the Roman Empire has led to dozens of other titles that chronicle the decline and fall of all sorts of other things. Although the trend is most prevalent in adult books, children's publishers are not immune to it. Of children usually like repetition, consider the Nancy Drew titles The Case of... over and over again, or the Harry Potter titles which always open the same way. But books that use inspiration from one title to write another on a completely different topic should be looked at carefully. Borrowing the idea of a title is all right, but we want to be sure the ideas inside the book aren't borrowed, or derivative, as well.