There are many books on the Seerah (biography) of Prophet Muhammad (SAW). Some of them are very thorough and detailed, for example Sirat-un-Nabi has five volumes. You can find a list here.
To start, Ar-Raheeq Al-Makhtum is one of the greatest. This book won the first prize in the first Islamic Conference on Seerah, in 1979. It is originally written in Arabic, but translated in Urdu by the author himself. I have seen the English translation as well, but a friend told me that the English translation is not as precise as the Arabic or Urdu versions are.
If you click here, you can find the PDF version.