First we have of course Darwin's own autobiography, which can be found for free online here: http://darwin-online.org.uk/content/frameset?itemID=F1497&viewtype=image&pageseq=1

For a comprehensive biography, look for Janet Browne's two-part work Charles Darwin, a Biography (Jonathan Cape, 1995 - 2002)

For a more concise and palatable biography, try The Reluctant Mr Darwin by David Quammen (W. W. Norton, 2007).