This was the first published biography of Charles Darwin, as edited by one of his children. It's a little unusual in format, containing as it does, Darwin's (intended to be private) autobiography, a pile of Darwin's letters (up to 1860, the year Origin of Species was published), some memories from the editor and a final chapter by Huxley on the reception of Origin of species, from the perspective given by a 30 years' interval.
Previously I've found letters collections to give a lot of insight into the character of their authors, more so then their biographers do. The letters here make clear Darwin's conflict between his egotistical desire for "credit" and his Christianity-derived value of humility, which wins out in terms of his behaviour regarding Wallace. They also demonstrate Darwin's focus on scientific matters and his method of working as well as his ill-health through much of his adult life. I'm not sure I'd recommend this as a starting place for Darwin studies, though. Instead read The Voyage of the Beagle and Bowlby's biography first, (bearing in mind that the latter's Freudian theory of Darwin's character is less convincing than the theory that Darwin was Aspie, though).