As you might have heard by now, I'm working with Jeff McAffer and Paul Vanderlei on an OSGi book. The book will be published by Addison-Wesley and will be part of the Eclipse Series. The working title is Equinox and OSGi: The Power Behind Eclipse, and you can even find it listed by Amazon, but the book won't be available until 2009.
The book is firstly an OSGi book, and secondly an Equinox book, and it covers many of the best practices that we have learned from building real-world OSGi applications. The bulk of the book is a tutorial that develops an application as you proceed through the chapters. There's also some deep dive chapters that attempt to discuss more complicated issues in detail.
We've just submitted some chapters to be published online under the Rough Cuts program, so be sure to check it out! There's also a blog dedicated to the book that we intend to update as we get closer to the publication date.