This is a very interesting article in the New York Times about the books that Mark Twain owned, and his use of marginalia (the art and practice of writing commentary on the pages of a book in the margins, or the top and bottom of the pages) to execute literary criticism.

All self-described nerds and geeks (myself included), and those who love literature and the power of the written word, will find a kindred spirit in the persona of Mark Twain–not the author, but the person who respected and cherished books as well as cherishing his own reaction to that which he read.

New York Times Article on Mark Twain