I am currently reading
this, which I got for my birthday the other day, basically because I have a huge hard-on for evolution, but also because I've always been fascinated to know how the man himself felt about the importance of his work, and the significance of publishing a book which basically undermined one of the Christian faith's major claims to validity.
It's very well written, so I'm ploughing through it at quite a pace, but it's definitely focussed on his life rather than his science, so I think I'll definitely look out a more scientific-based account in the near future.