Descartes’ own letter to his French translator explains what the work is for, and then come the Principles themselves: the foundations of human knowledge, beginning from the certainty that I think, therefore I am, followed by the nature of material things, the visible world and the earth. John Veitch’s translation gives the parts most often read.
The most systematic statement of Descartes’ philosophy and physics, in a manageable selection. Download the free EPUB.