Descartes' Secret Notebook: A True Tale of Mathematics, Mysticism, and the Quest to Understand the Universe
Biography, mathematics, philosophy
I've enjoyed many of Amir Aczel's books, but this one was a letdown. Mostly it's a capsule biography of René Descartes, written in young-adult prose. The titular notebook is only discussed for maybe twenty-five pages out of 200+, and its contents don't prove to be tremendously revelatory. The explanations of Descartes' union of algebra and geometry were good; there should have been more of them, though. Aczel also wastes a lot of time on what seem to me to be decidedly peripheral questions--whether Descartes was a Rosicrucian, for example. I expect the book would be better for younger readers, or readers with virtually no familiarity with the subject matter.
The same author's Pendulum: Léon Foucault and the Triumph of Science is one of his many good books. For a good Descartes book, try Russell Shorto's Descartes' Bones.