The book does not change, so anything different is you. Jokes you missed, a character you thought was annoying who now seems sad, a whole layer of meaning that was sitting there in plain sight. That difference is one of the few honest measurements of growth available, because most of the ways you have changed are invisible from the inside.

Pick something you loved at twelve and read it again this month. Notice what you see now that you did not then. Do the same thing in five years and you will have a strange and useful record of yourself.