The PPC and AdSense Book.
I have always seen having a good site out there in cyberspace as like having a piece of real estate. Its all about location, location, location with real estate and all about content, content and content in cyberspace. In cyberspace you can own a shop on the main high street and compete against the big boys, and win. Just like any shop, if you dress the window good, they will pour through the doors. And this is where Google's PPC AdSense comes in. Maybe they don't see what they want and click an AdSense PPC link that catches their eye. Or maybe they trawl through your pages to visit other things of interest and click an AdSense link that looks interesting. Or, if your site is really good, visitors will click other contextual affiliate links and you get a percentage of the sale value plus, they will keep returning and tell others. Each visit increases the odds of an AdSense click or sale. While on the high street you might expect a sale from 50% of visitors entering the shop, in the cyberspace adverts shop it averages at around 4%. But unlike the high street shop, the cyberspace one can handle thousands, even millions of visitors each 365 days. Now 4% of that at anything from 10c to $10 per click, is more sales than you could ever make from a high street store and hardly any worries or overheads.
There is no limit to the amount of cyber-property one can own, your only restriction is that imposed by the amount of hours one can physically work, so automating as much as possible is vital.
In a nutshell. An advertiser may have spent fortunes on developing a super site to sell their super product and promote it with Google AdWords. But if the content of your site is useful about that product, it could be your site, not theirs that will be top of the search engine pile. Your visitors see their advert on an AdSense link and wallah, you just made a sale. The advertiser is delighted with your referrals and bids higher to get more of their keywords on your site, which means you get more per click. That's the interactive beauty of Google AdSense.