The Open Data Handbook, published by Open Knowledge (Foundation) a couple of years ago, has been a popular primer for everyone wanting to be introduced to the term in general as well as a resource for more experienced professionals in the field. However, as the development and application of open data keeps making quantum leaps all over the world, an update of the book has been needed for quite while - up until now. My colleague Mor Rubinstein has been leading the work to thoroughly update the book, strongly aided by another colleague of ours, Katelyn Rogers, and the result has just been published - and, in my humble opinion, really hits the nail on the head in capturing the what, where and why of open data and the essence of it has come to. It features introductory information, definitions, case studies and much more - and is, of course, freely available under a CC-BY license.
As a side note I'm honored to say that I've also contributed a little bit, by co-writing bits in conjunction with Mor's work on the article titled "Open data business - an oxymoron or a new model?". Read it here - and you can see the (rather humbling) full list of great authors here.