Michael W. Lucas is working on a DNSSEC book that he’s self-publishing, similar to SSH Mastery.  He’s making an early draft available for purchase, at a discount.  You get access to the updates, so you effectively get the book for less, plus you can offer feedback before the publishing date.

This is a familiar concept for software, where early purchasers get access to a ‘beta’ version of software for testing…  It’ll be interesting to see how it works for a book.