The Long Tail is a theory by Chris Anderson turned book. The actual theory is very interesting and explains how our economy is going from one of a few hits to one of thousands of niches. This set of niches while individually small, collectively make up a large amount of revenue. He explains this in the idea of the 98% rule.
Consider Rhapsody, the online music store with over four million songs in it's library. What percentage of the songs are sold at least once a quarter? 98%. What about the percentage of books sold on Amazon.com? 98% of all books listed on Amazon are sold at least once per quarter. This is possible with the advent of the internet. Once the means of distribution drop to 0, choice shoots up to almost infinity, and everyone can find their own specialized niche creating a long tail. Once the means of production drop to 0, everyone can create something inventing new markets along the way.
While the actual theory is really interesting and very insightful, the book could've been cut in half. After the first few chapters, the whole book keeps repeating itself with different examples with a little bit of new information to keep you going. He explains many times over how the means of production and distribution are dropping to zero and what it means. This allows you to crowd-source information such as in astronomy or wikipedia. I don't know why it takes him forever to explain it. Overall though, the original Wired article is a must read. Buy the book if you want more details and examples, which was too repetitive for my tastes.