Descartes’ Secret Notebook by Amir D. Aczel was thoroughly enjoyable. Even if mathematics is not your primary interest this book is still a fascinating piece of history. Descartes lived in fear of the Inquisition from the Catholics, but having been born Catholic he lived equally in fear of persecution from the Protestants. When he was young he also had serious health problems, so there was no shortage of adversity. Yet an amazing intellect was more powerful. Through the many journeys and associations of Descartes one may learn much about the Renaissance world.
From the perspective of a scientist and mathematician it is delightful to see the relationships between Descartes and many of his contemporaries including Kepler, Newton, Leibnitz and others. It appears that Descartes’ may have been on the verge of discovering calculus but feared the heresy of discussing infinity. Additionally he had insight into the future work of mathematicians such as Euler.
Descartes appears elsewhere on this site, namely on the page for Cartesian Battleship.