Until I read this book, my entire knowledge of Rene Descartes consisted of the stanza from Monty Python's Philosophers Song. Accordingly, I approached Descartes' Secret Notebook by Amir Aczel with almost no preconceptions about Descartes.
The title refers to an encoded notebook found in Descartes' possessions after his death. A student philosopher, Leibniz, was allowed to copy some of Descartes' papers, including a few pages from the notebook. A few years later, the notebook was lost and was never seen again.
The majority of the book is a somewhat cursory biography of Descartes. As the actual notebook was only 16 pages long, and the only remaining copy is just a few pages of that, it would be difficult to spend an entire book on the notebook.
Instead, most of the book follows Descartes in his travels around Europe. The biography works to explain Descartes' intense need for secrecy and generally succeeds. Descartes lived in a turbulent, dangerous time when studying physics and math could bring down the wrath of the Catholic Church upon him. Accordingly, he delayed publication of some of his books, and significantly altered others in an attempt to avoid persecution.
The author returns to the notebook at the end of the book, and discusses what was found within. This part is apparently somewhat controversial, judging from some of the angry reviews I found on Amazon. I found it plausible and well-explained, and the author ties it in with some current cosmology theory in an interesting discussion.
Overall, I enjoyed the book. If I were more familiar with Descartes, I probably would have found the biography brief and lacking. As it was, I found that the book served as an excellent introduction to Descartes, and got me interested in finding out more about the time period.