Muhammad: His Life Based on the Earliest Sources, Dr. Martin Lings

Dr. Martin Lings was a close friend of C.S.Lewis when they were in college. Lings was trained as a Shakespearian scholar, but he became interested in Islam and converted in the 1940s. His biography of Muhammad is one of the most widely accepted biographies in the English language. 

I highly recommend this book because it portrays the life of Muhammad from the point of view of a true Muslim believer. Much of Islamic faith and practice is based on the life of Muhammad. Lings does not gloss over things like Muhammad's marriage to Aisha when she was six years old (consummated when she was nine). He definitely shows Muhammad's great involvement in warfare and that in Islam, state and religion are "inextricably bound together." 

I have written a brief comparison between Christianity and Islam that draws from some of the things presented in this book.