I’d like to support you more by buying the book, but I have no clue whether it is available in Germany. Do you have any info on that?

Yep! It’s available here I’ll be posting more info soon.

Do I have to buy something from the sites that I click on or is it enough that I just check them out?

Checking them out is all you need; buying something is up to you.

Though, for Amazon, you would need to buy whatever it is you want

Yes the unfortunate thing about Google Adwords is the tendency to have them linking onto the keyword “religion” and serving a load of Christian websites. I once wrote an article about the movie Expelled, only to have an advert for the movie displayed as a massive tower banner down the right side of the page

I think your idea of “supporting” them by clicking is slightly backwards though. You aren’t going to be swayed by anything they have on their site, and the more clicks they get through adwords, the more they have to pay, so in fact you are making them pay more money that isn’t getting them anywhere.

As for the rule, it is quite a common one in the web advertising world because the point of adverts is to catch the attention of the viewer for content, and for them to be clicked out of interest, not for profit.
A notable exception is “Text Link Ads” which are different since there is an active contract between you and the sites that participate, meaning they have to pay you a monthly fee. To get more money however, I have signed up to a premium account which means I cannot display any text relating to the adverts on the website (not even a headline saying “Adverts”). This is why on the homepage of my site I have links to a Psychic and Prayer Requests. Sure, it looks hypocritical (especially when I can’t explain it on the blog), but it brings in more money which is needed to pay for the server my site is hosted on.

Anyway, I’d guess a fair proportion of your adverts, carefully targetted by Google AdWords, would come from evangelical Christian sites? By clicking through I guess it would transfer money from them to you, but I still can’t bring myself to “support” them by adding to their web hits this way.

Google disallow people asking viewers to click on adverts. I’d take down your message if I were you, since they also have a habit of closing advertising accounts for the smallest of rule breaks.