Memories.

“Isn't it odd how much fatter a book gets when you've read it several times?" Mo had said..."As if something were left between the pages every time you read it. Feelings, thoughts, sounds, smells...and then, when you look at the book again many years later, you find yourself there, too, a slightly younger self, slightly different, as if the book had preserved you like a pressed flower...both strange and familiar.” 

“If you take a book with you on a journey," he had said when he put the first one in her box, "an odd thing happens: The book begins collecting your memories. And forever after you have only to open that book to be back where you first read it. It will all come into your mind with the very first words: the sights you saw in that place, what it smelled like, the ice cream you ate while you were reading it... yes, books are like flypaper—memories cling to the printed page better than anything else.” 

Cornelia Funke. from Inkheart