Although there's plenty written about free software and open source, there's relatively little in the form of books that try to offer a synoptic view. This makes the annual Open Source Jahrbuch, particularly valuable. As for 2004 and 2005, this year's is freely available as a PDF.
As you might expect, it is planned with a Germanic thoroughness, weighing in at 500 pages. As well as big names like Eben Moglen and Larry Lessig, it has a host of less well-known writers, who nonetheless have interesting things to say. I particularly liked the details of the famous Munich LiMux project, and the corresponding project in Vienna, WIENUX. Also good is the article on open source community building, which analyses several smaller projects.
I was pleased to see plenty of space given to both open content and open access. As readers of this blog have heard ad nauseam, there exists an important commonality between these opens, and it's gratifying to see open source's younger siblings getting some recognition here.
All-in-all, I'd go so far as to say that this is the best book on open source that has been published in the few years or so. Taken together, the whole series of Yearbooks form perhaps the most important collection of writings on open source and related areas to be found in any language.