Charles Darwin was, among many things, an avid reader, and by the end of his life he had created his own small library. Not only did he read a lot, but he also compared his ideas with those proposed by the authors, which becomes clear when noticing the copious amount of annotations he added to the texts he was reading. And now, everyone can see these. The most heavily annotated works from Darwin's library have been digitized by the Biodiversity Heritage Library. 

(Source: Biodiversity Heritage Library)

Browsing through Darwin's books, a helpful window is provided where his - at times illegible - annotations are transcripted.

Will this teach us anything new? Probably not, but it does provide a glimpse into the mind of Charles Darwin, a careful, attentive reader that critically, yet respectfully acquianted himself with a broad range of subjects, which has helped shape a little theory you might have heard of. 

Now, go and build your own library (digital or otherwise).