They are already reaping big checks from using adsense and I know a bunch of them such as Joel. The reason for writing ebooks as mentioned, they use it for two reasons: 1. When you have a niche product such as adsense. It will always be in demand as long as Google continues its adsense program and with thousands of affiliates selling his ebooks, guess who's laughing all the way to the bank. That's right. the owner of that adsense ebook. 2. They easily build a list of members and as the old saying goes, "The money is in the list". So mo money, mo money, mo money. I would do the same thing.
Just want to add to the discussion to agree with what most have said. Those who try to teach you techniques about how to make money using adsense are simply using forum links to boost the PR of their sites. These people are most likely not raking in the adsense loot. Most that make GREAT incomes with adsense don't have time to post in forums about their experience, nor do they want to give up their trade secrets. Also, those that say they're aren't adsense secrets are wrong. There are definately SEO secrets out there that people keep hushed. Every once in a while someone will out one of these SEO secrets (to the dismay of all that all ready had capitolized off of it), but there are still many things seo experts do to get their sites to rank higher that 90% of the industry doesn't know about. I also agree that if you're going to find out some of these secrets, this is definately the place, among a few others, to do so. I've learned a ton of things.