He has sponsored ads on the blog plus he solicits donations. I have a small blog in the same field, though with a lot less traffic and that site makes a nice side income on under 20K page views a month. From his Alexa rating he is getting 1,000,000 page views a month in the financial field which is pretty lucrative. So he should be getting a pretty penny form those ads. The pictures on the forums are actually ads, too.
Check out
Online Advertising Expenditure Forecast, 2009-2010 | ClickZ. "Online advertising, which includes display, search, video, and other categories, is expected to tally $25.4 billion in spending this year and $26.1 billion in 2010."
Not all web sites can make high rates so not every site in the top 50K is going to get the same rates. A joke web site would probably make 1/20th, if that, per impression as a financial site. It depends on the topic.
Maybe as an additional revenue stream? If he can live so cheap and doesn't need any more money then why write a book at all? Best selling books can make millions for their authors and allow them to reach people other than simply a blogging audience. With a book you can get on TV and radio, maybe even Oprah. Look at how many books the original 4 hour work week guy has sold.
He is a good writer and he has some interesting ideas, but I think he is too smart not to be making a ton of money off his site and now his book.
Plus I do question some of his claims. I have only just skimmed his sites but it seems he he hangs out in Berkeley but lives in an RV and doesn't have a car? Is that right? I don't know where you would park an RV around Berkeley. It is in the SF metro area. To park an RV I would think you'd have to live farther out of town where land is cheaper but then you would not have easy access to mass transit to get to places like the Berkeley marina.