As a biologist I have really enjoyed reading a lot of older scientific books which one can find on
Project Gutenberg. Lately I've been reading the works of
Alfred Russel Wallace, whose work helped develop the theory of evolution and natural selection.
I recommend reading
Contributions to the Theory of Natural Selection: A Series of Essays most of all. It's fascinating material not only for the direct subject matter but also for the historical perspective it provides. It's a wonderful glimpse into a past time of great exploration, debate, and unique culture. Best of all, it's amazing to see how it all fits into our world today.