The Origin by Irving Stone
I just got finished re-reading The Agony & The Ecstasy by Stone a few months ago and enjoyed it immensely (again). So I checked out The Origin , about Charles Darwin's life. His voyage in the Beagle is very interesting, but after he gets back to England, it's really quite a slough. He did a lot of writing and meeting naturalists/geologists/scientists (in addition to having lots of kids), which doesn't make for quick reading. It seems, at least in this novel, that Darwin had a lot of advantages in life and I think one of the reasons I'm not as keen on this book is there is not a lot of adversity (internal or external), unlike The Agony. He's finally gotten to his ideas of natural selection, so I'm looking to see where that's going. Overall it's a quality read, but probably not a book I would pick up again (unlike Agony)
Patrick O'Brian The Aubrey-Maturin series
This long series are my favorite historical novels. The series concerns a British sea captain and his surgeon during the Napoleonic wars. These novels are dense and the language takes some getting used to as it is written in the early 1800's sea vernacular. I bought an unabridged dictionary for these novels. This series is a proper fit for readers who like military history, 19th century English lit or sailing.
My friend who turned me on to them has read the series several times and one college friend is now rereading the series. It will likely take you a couple of years to read the entire series. The movie Master and Commander is based on one of these novels.