In his Meditations Descartes tries to discard everything that he thinks he knows about what he is, and builds from the foundations up using his famous methodical doubt. Taking in what it is to be human, the Cogito principle, Cartesian duality, the existence of god, and the nature of error, he finally reaches a point where he can be secure about what it is possible to know in a clear and defined way.
Written in 1641, Descartes didn't have the full support of the theological colleges, and this edition helpfully attaches the Objections made to his Meditations, and Descartes' own replies.
For us, the kind of circular reasoning that Descartes demonstrates might not stand up, and certainly we have a very different view of the connection between mind and body following neuroscientific knowledge and ongoing research. But Descartes is important for the scientific methodolgy he tries to instil, and the fundamental nature of the questions he asks, even if some of our answers might be different.
His style isn't always the easiest to understand, as he tends to ramble and be quite repetitive - but for anyone wanting to get a handle on either modern philosophy or the Enlightenment and the ideas which followed, Descartes is fundamental.