One book I am relatively familiar with is "The Venture of Islam" by Marshall Hodgson, however this is source is not, as you requested, strictly about his life but more about the context and influence of Islam. It is encyclopedic in nature and quite thorough; however the section on Muhammad himself is rather short and gives little insight into his personality etc.

A professor of mine recommended to me Muhammad: His Life Based on the Earliest Sources by Martin Lings as perhaps the most comprehensive biography of Muhammad and very readable as well. I regret to say I haven't gotten around to reading it yet so I can't recommend it personally but I think it would fit your criteria a bit better.