What is the best guide to learn about Google Adwords? I was talking yesterday to a fellow who runs a lot of campaigns. He was explaining to me that changing your ads can increase clickthrough by 30x! Especially if the search keyword is in the ad. He also mentioned some things that I didn't know like: * Low competition keywords can actually be really expensive. (Why?) * Google gives you a good ad score if your ad has high clickthrough, and the landing page contains the keywords. On the downside, high clickthrough means that the traffic is less targetted and has worse conversion. I'd like to read a guide that explains to me these concepts in more depth. An in-the-trenches guide to adwords. What do you recommend?