Reddit mentions of Life in the Universe: A Beginner's Guide (Beginner's Guides series- Astrobiology)

If you want a good introduction that's not too expensive or complex to ease you in, I'd recommend http://www.amazon.com/Life-Universe-Beginners-Guides-Astrobiology/dp/1851685057/ref=sr_1_1?ie=UTF8&qid=1344106799&sr=8-1&keywords=lewis+dartnell

This is a very broad course textbook style introduction as well http://www.amazon.com/Introduction-Astrobiology-David-Rothery/dp/1107600936/ref=sr_1_2?s=books&ie=UTF8&qid=1344106857&sr=1-2&keywords=astrobiology

I've heard good things about the Plaxco one though.

In the end though, I'll say what I've said before - 'astrobiology' isn't really a thing yet - it is a context for particular types of science. I would look into what specific type of science you would want to be doing that would fit into astrobiology (say, geomicrobiology, or whatever) from one of the introductory books here and then start reading up on that.