Some of the best topics I found on Decoupling Capacitors:

1. Decoupling Capacitor definition and working principle on Wikipedia:
http://en.wikipedia.org/wiki/Decoupling_capacitor

2. Filtering Techniques: Isolating Analog/Digital Power Supplies in PLL-Based CDC

http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=scaa048

3. Power Supply Integrity - Altera
http://www.altera.com/support/devices/power/integrity/pow-integrity.html

I hope readers looking more information on decoupling capacitor will find these links useful.