In short, the astronaut Mark Watney is stranded on Mars, initially thought dead, and must struggle to survive with limited resources until the next Mars mission arrives. The first problem: he has enough food for several months, but not nearly enough to last four years, until the next Mars mission is scheduled to arrive. A smaller problem: his entertainment is mostly limited to disco and seventies sitcoms.
As I read this, I was struck by how much it felt like I was reading a web novel. Perhaps, I thought, the style of novels has just changed that much--I haven't read many published in the last decade. After I finished, though, I found the truth: the thing was self-published as a free story on the web before it was put together as a book. I had no idea, but it seems my sense of style is well-adjusted!
The book was fun. Mostly just that--fun. It's an adventure story, basically. It doesn't use Mars as a metaphor for examining any deep truths, but it doesn't pretend to, either. An easy but worthwhile read.