I like to guess the "timeless-ness" factor of a book. If a book seems to have it, I'm likely to shelve it to build a library for my kids. Filter question: will this book have any value in 10 or 20 years from today?
A late theologian from my country, truly wonderful and witty academic, used to say something like: it would be insulting for a book if you grab it from a store and start reading right away. A well-mannered, civil reader will first let the book mature in the shelf for a good ten years or so. :)
This was said tongue-in-cheek, obviously, but it's a good line. She was a classical philologist, translated works by Platon, Hieronymus, Martin Luther etc. So I suppose she lived in a somewhat different time zone as compared to the rest of us -- as in, if your mental horizon expands to some 2000 years, a decade of waiting for a book to "mature in the shelf" is, indeed, nothing. :)