The e-books are crap. They're just making money off of people's false hopes.
Adsense is a great way to monetize sites with content. It's so simple to use and Google technology guarantees the best, most relevant ads. It's worth displaying some Adsense on every site you own.
However, be warned: it's not easy to make real money with Adsense. As mentioned above, it takes time and often a lot of work. And in the end, you can never be sure whether you project's gonna take off or not.
Developing minisites with adsense can easily fund a domain's own renewal costs, as long as you make about $1 a month, and with adsense that's not hard to do. All you need is some relevant content and some links from other sites with traffic. Content you can write or buy, as mentioned above. I've used getcontent.com and they're good enough for minisites (example:
http://www.distancelearningmba.org). Considering their cost you need to make about $30 in the first year to break even. Not too hard. If your domain is any good, you'll make enough to fund a few other domain purchases as well.
Making real money with adsense is a different matter. I have one site that I've developed for 3 years and now it's making about $400 per month. It's got about 600 pages of original content (written by me). That's a lot of development and work for a meager return. It's not easy to find Adsense gold. My other sites are bringing far less than that. The good thing is, and here's the beauty, the revenue keeps rolling in day and nite even if you quit updating the site. If the site is properly SEO'd, organic traffic will be coming in long after you have moved on to different projects.
Adsense is strictly for content-rich sites. Clickthroughs are great with targeted, quality content, but it's not much good for web tools and other sites without much textual content. For instance, on my EstiBot I rarely ever get any clicks. The click-through rate on my content-rich site is high, but on EstiBot it's dismal. Affiliate links may be better for that type of sites.
It's definitely worth signing up and experimenting with adsense. If you already have sites with traffic and content, you're missing out by not displaying adsense ads. Almost all websites with content these days seem to display Adsense. That's why Google is so rich
Good luck
Josh
EDIT: I just read the blog mentioned in Damon's post and want to point out that I've never experimented in that sort of thing. Looks like it could work, but you'd really have to get into it. I only speak from the experience of creating original sites and relying on organic traffic.
