Random Access :: Fitness matters
Does keeping healthy really increase a person's ability to perform on the job?
Productivity level almost always map to your physical fitness level at any given time.

Freakonomics :: by Steven D. Levitt and Stephen J. Dubner
This is an insightful book that highlights the learnings to be gained when you apply one science (economics) to another (sociology).
The book poses and answers some interesting questions such as: "Which is more dangerous, a gun or a swimming pool?" and "Why do drug dealers still live with their moms?"