Espresso: Ultimate Coffee by Kenneth Davids. A good beginners book on espresso preparation. I recommend reading this book before reading the more advanced book below.

Espresso Coffee: Professional Techniques by David Schomer. This book is currently the best book available for describing espresso preparation. I recommend it highly for any barista or coffee shop owner. The price is inexpensive, but some of the techniques are difficult to learn without watching somebody. Please check the espresso section of this web site for details on some difficulties I had implementing some of the techniques. In the meantime, order this book!