C. S. Lewis, letter to Arthur Greaves, 1932: 

To enjoy a book like [Froissart’s Chronicles] thoroughly I find I have to treat it as a sort of hobby and set about it seriously. I begin by making a map on one of the end leafs: then I put in a genealogical tree or two. Then I put a running headline at the top of each page: finally I index at the end all the passages I have for any reason underlined. I often wonder — considering how people enjoy themselves developing photos or making scrapbooks — why so few people make a hobby of their reading in this way. Many an otherwise dull book which I had to read have I enjoyed in this way, with a fine-nibbed pen in my hand: one is making something all the time and a book so read acquires the charm of a toy without losing that of a book. 

UPDATE: A terrific response to this post by my buddy Austin Kleon.