As part of the "Books That Changed the World" series, Darwin biography Janet Browne has penned a short "biography" of Darwin's Origin of Species (forthcoming from Atlantic Monthly Press). Part overview of Darwin's life, part discussion about the book itself, and part an examination of the impact the Origin has had in the almost 150 years since its publication, Browne's book is both a decent introduction to the text and to Darwin's thinking.
The book's size is its most limiting factor. Undoubtedly constrained by the strictures of the series, Browne is unable to provide much more than a cursory glimpse at the convoluted and intense process by which the Origin came into being, not to mention the man behind that genesis. Its post-publication impact is assessed more thoroughly - Browne has done an admirable job reducing more than a century of scientific thought into forty pages or so.
There are more complete examinations of both Darwin and his works available, but for a readable and succinct survey, this is a good place to start.