Great short article by Austin Kleon on marginalia, including some excellent examples of the practice taken from Oliver Sacks' library. Found via this post by Doug Belshaw, his blogging itself is very much like marginalia.
I believe that the first step towards becoming a writer is becoming a reader, but the next step is becoming a reader with a pencil. When you underline and circle and jot down your questions and argue in the margins, you’re existing in this interesting middle ground between reader and writer. […]
I love this idea of marginalia as a way to turn a book into a medium for conversation — a kind of literary note-passing.