Ever re-read a book you once read a long time ago, only to find your view of it has fundamentally changed in some way? Perhaps you identify with different characters in a story, or your understanding of the author's arguments is entirely different. It's a strange experience, since it's the same book - but you have changed, and so the book has become a kind of time mirror that shows those personal changes reflectively.

(It was Albert Camus' The Plague for me, revealing something of a transition from hopeful idealism to rather jaded realism).