Muhammad: His Life Based on the Earliest Sources, by Martin Lings (Abu Bakr Siraj ad-Din). This book is primarily based on Sirat Rasul Allah by ibn Ishaq (recension by ibn Hisham) with supplemental information taken from the "six sahihs" hadith collections, the biographies of ibn Sa'd, and the history of Tabari. It is written in a wonderfully clear and engaging style.