Elika Ortega in a talk at Experimental Interfaces for Reading 2.0 mentioned two web sites that gather interesting material traces in digital books. One is The Art of Google Books that gathers interesting scans in Google Books (like the image above).
The other is the site Book Traces where people upload interesting examples of marginal marks. Here is their call for examples:
Readers wrote in their books, and left notes, pictures, letters, flowers, locks of hair, and other things between their pages. We need your help identifying them because many are in danger of being discarded as libraries go digital. Books printed between 1820 and 1923 are at particular risk. Help us prove the value of maintaining rich print collections in our libraries.
Book Traces also has a Tumblr blog.
Why are these traces important? One reason is that they help us understand what readers were doing and think while reading.