I'm reading Darwin's Origin of the Species.
I've read extracts before, and one day I sat in a library and read a chunk of it, but I've never actually read the book before.
I'm not doing anything high-minded like reading it for the blog for Darwin campaign or anything - it just happens to be one of the many books I bought when I was in Washington D.C. last year, and I've read the other science books (Shubin's Your Inner Fish, Sagan's The Varieties of Scientific Experience, both recommended). I just wanted to read it, and there was a reasonably-priced edition.
I feel the same sense of excitement I had when I plunged into D'Arcy Thompson's On Growth and Form, a book which can unfortunately only be experienced for the first time once. (If you've never read it, do yourself a favour and try to find a copy. Mine was the shorter 328 page edition.), except that with Origin, instead of a well-regarded descriptive book on the fringes of mainstream biology, we have a book critical to modern biological thought.