The first time I saw this quote was in Cornelia Funke's Inkheart (fantastic book, by the way) on the door of Mo's workshop. For those of you who have not read the book, the story centers around a young girl named Meggie who loves books, and her father, Mortimer, who repairs and restores them. He keeps this quote on his workshop door, and when Meggie was young, she was horrified because she thought he was referring to actually eating books :)
"Some books are to be tasted, others to be swallowed, and some few to be chewed and digested."
-Sir Francis Bacon