TidBITS, a venerable Mac newsletter, has launched Take Control electronic books. They're short, focused, timely, and only cost $5. They're designed to cover the middle ground between long web articles (often hobby-written, aimed at too technical an audience, and too short) and weighty 500-page tomes (too bloated with stuff you don't need, and too long a production cycle to get the info you need now). It's interesting to see the spectrum of publishing options get populated with more product points.