A. Boyce Gibson 
The Philosophy of Descartes [PDF ebook] 

Maintaining that it is impossible to understand the work of a philosopher without understanding the previous history of thought and the contemporaneous developments, this book, originally published in 1932, is an in-depth study of Descartes’ philosophy with a strong emphasis on the historical approach. It covers Descartes’ early life and education, before continuing to discuss his method of doubt, the existence of God, the scientific interpretation of nature, the unity of knowledge, the attributes of God and free-will.