Skimmed this in a Barnes & Noble while I had time to kill. Basically it's the product of some scarily obsessed fanboys. It includes a fanatically comprehensive set of interviews -- they not only tracked down the actor who played Larry, the kid who steals the Dude's car ("IS THIS YOUR HOMEWORK, LARRY?") but also the former kid whose real-life involvement in a car theft inspired the Larry incident. They even interviewed the actress who played the supermarket checkout girl in one of the opening scenes. And many others (they did not succeed in interviewing the Coen Bros., however). There are a lot of background stories about where various other ideas come from.
The book's mildly entertaining, but it doesn't come close to the things I really want to know about the movie, which mostly have to do with its repeating themes: a traditional film noir structure, complete with femme fatale and hostile cops, revolving around a bewildered stoner rather than a hard-boiled detective; dialog that gets picked up from character to character ("This aggression will not stand, man!"); constant references to belief systems ("Fuckin' nihilists... say what you will about the tenets of National Socialism, at least it's an ethos").
I may be overthinking this, but I'm convinced there's something going on there under the main story. But you'd probably have to interview the Coen Bros. to get at that stuff, and apparently they're not talking. ("No, Walter, it did NOT look like Ethan was about to crack!")