Darwin's Autobiography serves as a good overview of his life and the major events that happened to him. While the actual autobiography itself is very short and lacks details, its a good starting point for someone wanting to learn more about Darwin. In this edition edited by his son Francis Darwin leaves out some passages about Darwin's family and married life, something one could argue as particularly telling or interesting information; if this bugs you, buy the later edition.
One of the most interesting sections to me was Darwin's description of his boyhood and young adult years. It's comical to hear this scientist describe his obsession with the pastime of shooting things and his mediocre performance in school. A few things signal Darwin's observational powers or scientific inclination, such as his collection of beetles, but for the most part, he seems an ordinary young person.
Also, the book continually references scientists and intellectuals of the time which Darwin comments on. Some of these people were close to Darwin, others he just mentions. Now knowing these people can be somewhat frustrating to the reading, as I can attest to. The book is very much written and directed at his children, who would be familiar with this social context.
Even with these minor faults, Darwin does give insight into his own mind, something I'm sure anyone who's reading a book about Darwin is looking for. The introspection comes at the end of the book. Darwin speaks of his own reasoning capacities and ability to notice things which easily escape the observations of other men.
This book is short and a I recommend it as a good place to start for getting a handle on the major events of Darwin's life and hearing Darwin's own perspective.