One of the very best O’Reilly books I’ve read in a long time, is Developing Feeds with RSS and Atom by Ben Hammersley. On 240 odd pages, the author introduces the reader to syndication, feeds, the RSS dialects, the Atom syndication format and discusses parsing of feeds and creating feeds. This he does concisely and to the point but with a sense of humor which I greatly enjoy.
The only thing which might be expanded on in a second edition, would be how different RSS reader applications react to feed content. Never the less, I highly recommend this book to anybody wanting to get inside RSS feeds.