Also, Richard Feynman’s remarks on the loss of the Space Shuttle Challenger are a pretty accessible overview of the kinds of dynamics that contribute to major industrial accidents. http://history.nasa.gov/rogersrep/v2appf.htm
[edit: corrected, thx.]
A good quick-and-dirty test uses the humble push-up. Periodically (every two or three days) just do as many push-ups as you can—this will likely involve moderate discomfort on the last few—and track the number you do over time. While there is some day to day variance, I think this is a pretty good rough proxy for general fitness and a few weeks of data would give you decent tracking of the trend, unless you are already in such good shape that marginal improvements are hard to discern.