Just a word of wisdom to those of you playing around with your O2
sensor meters... be sure to install them (the O2 sensor) BEFORE the
catalytic converter (if so equipped).  Otherwise, all you are
reading is diddly-squat in terms of meaningful data.

2 great books on O2 sensors/fuel injection are (info posted earlier 
but what the heck):

Fuel Injection and Engine Management by Charles Probst

How To Tune and Modify Bosch Fuel Injection by Ben Watson