An Introduction to Astrobiology

I am doing some readings about aliens this summer and this was one of the books on my list. It is basically an undergraduate level textbook with some fairly nontechnical chapters on the genesis of life, cell biology, the essential chemical reactions of life, extremophile terrestrial species, planetary geology, exoplanet astronomy, and SETI. It does a very good job of explaining what we know and how we know it. The book is about five years out of date (most relevant for the astronomy section, as exoplanet discoveries are starting to pour in) but gave me a good basis for further reading on the subject.