I stopped the Google Adwords Express campaign a month ago, and found no drop in sales. When I tried restarting the campaign, it was far more expensive. Whereas before the cost-per-click was around 20-30 cents, now it is over $1 per click for the same ad. In addition, I found minimal change in sales. I believe that the ads helped to develop initial organic growth, and now they are too expensive to continue using.
As a result, I'm running campaigns on various book promotion websites. I plan on doing these promotions in a "throw a bunch of noodles on the wall and see what sticks" approach. I'll keep you posted on the results!