Michael Cook's Muhammad is a good place to start even though it was written in the 80s - it's both short and readable! A newer, similarly introductory book would be Fred Donner's Muhammad and the Believers, but it covers early Islam as a whole and is pushing for a specific argument; it's still good if you are looking for some contextual information for Muhammad's life though.

Just for reference the general conclusion of the current scholarship is that the Sunna is not the way to learn about the life of Muhammad. There's pretty significant consensus an enormous amount of the traditional literature, whether the hadith or the sira were written by later generations at a level of accuracy that is impossible to determine. They were simply too far removed to be reliable particularly given how often their accounts contradict documentary fact or have similar problems to other religious texts in discussing miraculous events.

Some scholars believe that there is a core "kernel of truth" that can be assessed through various means, or believe that they were essentially written close enough in time as to not be easily rejected outright, but the sira and hadith are hardly considered to be the definitive source of information about the prophet that they might once have been.

For more on that I would see Hoyland's short(ish) essay Writing the Biography of the Prophet: Problems and Solutions.

Aside from the recommendations so far, if you want the traditional account, there's A. Guillaume's translation of ibn Ishaq, entitled "The Life of the Muhammad."

There's also Watt's traditionalist Muhammad at Mecca, and Muhammad at Medina, and the concise edition "Muhammad: Prophet and Statesmen."

Karen Armstrong, Muhammad: A Biography of the Prophet

I liked reading her History of God book. Her “Muhammad: A Biography of the Prophet” structered well and gives a good perpective of the time (7th century) as well as The Prophet’s life.