Al-Madkhal ila Dirasah 'Ilm as-Sirah an-Nabawiyyah

This book explains the science of Prophetic biography: what it encompasses, its different forms, sources, etc.... 

A very useful research to begin the study of the biography of the Messenger of Allah ﷺ and to understand the methodology behind the many books on the subject as well as the sources to use and be wary of.