Coming Back For More of the Same

I've been thinking, the past few days, what it is that makes a book worth reading twice. There are three types of books in the world: those worth reading twice, those worth reading once, and those not worth reading at all.

While most discussions about books tend to be trying to divide between those worth reading and those not worth reading, I'm wondering today what it is that distinguishes those worth reading once between those worth reading over and over.

A book that's worth reading once can have good ideas and good plot, but good ideas can only be novel once, and a good plot, while that can still be part of the appeal, it isn't sufficient to be coming back for, because once you've read it once, you already know what happened. What I'm wondering today is, what is it about a book that makes it worth reading again even though you already know what's happened in it, even though you already know what nifty ideas it has.

Here's a few: style, snappy dialogue, a love of words, scenes you want to savour. For example, I absolutely adore the proposal scene in Lois McMaster Bujold's "A Civil Campaign", and also the Nikki-refuses-to-come scene just before that. It's just so delicious I want to read it again and again. But what makes a scene delicious may vary from person to person.

Ideas?