I recently read Craig Walls' book Modular Java: Creating Flexible Applications with Osgi and Spring (Pragmatic Programmers). This book is a very detailed tutorial that walks you through setting up an application using OSGI and Spring with the help of Maven as a build tool.
If you aren't familiar with any of these technologies, this book will get you started, and quickly have you feeling like you have a basic grasp of the concepts and technologies.
You'll finish the book with a desire to learn more about the technologies, and understand the power of modular applications. As a tutorial, this is an excellent book. This is not an general guide to how to design with OSGI. There is some background on the frameworks, and some explanation of technologies, and also pointers to sources for more information, but this book is all about learning by building. If you pick up the book hoping to learn any details about the how and why of OSGI and Spring, two useful technologies, you might be disappointed. But if you like learning by doing, and working with running code, you'll enjoy and value this book.