I just finished reading the book "Secret Formulas of the Wizard of Ads". It is a collection of writings by Roy Williams and it is excellent. I highly recommend it for anyone who is interested in brand development and advertising, both online and off. You can find more about Roy Williams and his "Wizard Academy" at: http://www.wizardofads.com/
Here's a few great concepts from him on writing great ads:
use powerful words
be specific
repetition is key
create echoic lines
cause the listener to imagine themselves taking the action you want