What bookshelves are for:

Bookshelves are not for displaying books you've read -- those books go in your office, or near your bed, or on your Facebook profile. Rather, the books on your shelves are there to convey the type of person you would like to be.

HT: evangelical outpost

Mortimer Adler on How to Mark a Book:

I contend, quite bluntly, that marking up a book is not an act of mutilation but of love.

(HT: Tim Challies)