Books: Espresso Preparation

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. 

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

Espresso Coffee: The Chemistry of Quality by Andrea Illy. A fantastic advanced perspective on espresso. It has a scientific edge, but the information contained within is superior to most other books. Highly recommended.