Pay Per Click (PPC) advertising can be quite lucrative. The keys are to bid on literally hundreds of lesser-used keywords and keyphrases to keep the cost per sale down. Then add traffic strategically with judicious use of the higher-priced, high-volume keywords in your field.
Leading PPC Ad Publishers and Distributors
The two leading PPC ad publishers and distributors are:
Google AdWords provides a great deal of flexibility to advertisers. Here's where you need to start.
Yahoo! Search Marketing "Sponsored Search" (formerly Overture). You can get a $25 credit when you sign up with this link. While Yahoo Search Marketing doesn't give you as much flexibility as Google AdWords, it covers a lot of search engine you can't afford to ignore.
Secondary PPC Players
These serve the smaller search engines that don't get nearly the traffic that Google and Yahoo bring, but their PPC prices might be 10% to 25%, so if you're struggling with a tight budget, try here. The only problem is, since traffic is far less, you may be reaching the point of diminishing returns for your ad placement time.
FindWhatis the largest of the second tier companies. If you are looking for lower priced PPC, start here. You get $5 to open an account with them using this link.
ReachLocal will help local businesses manage a local PPC campaign. They also team with consultants to offer this service to local clients.
Yahoo Local Sponsored Search (formerly Overture LocalMatch) at this point provides great value, since Yahoo is a leader in local search. Get a $20 credit when you sign up through this link.
CitySearch PPC ads also provides wide coverage for those searching on Yahoo Local and other search engines.
Books
The best books I've found on PPC advertising are:
Andrew Goodman, Google AdWords Handbook: 21 Ways to Maximize Results (updated May 2005). He outlines a strategy that will actually work if you follow his instructions. I consider Andrew one of the industry experts in this field. He speaks regularly at Search Engine Strategies conferences, etc. (Self-published e-book)