Dr. Jason O'Kane has just published his book titled "A Gentle Introduction to ROS." The book is also free to download as a pdf. From the backcover:
ROS (Robot Operating System) is rapidly becoming a de facto standard for writing interoperable and reusable robot software. This book supplements ROS's own documentation, explaining how to interact with existing ROS systems and how to create new ROS programs using C++, with special attention to common mistakes and misunderstandings.