By their covers.

First class each semester I hand the students a questionnaire. One of the questions asks the students to draw, from memory, the cover of their favorite book of all time. I superimpose the drawings over a photo of a blank book, then show them to the students the following class.
Spine Out, “Favorite Book Sketches”

Related: The Times on books as decorations and the dying art of judging commuters by their reading preferences.