Also on my bookshelf is Programming Interactivity, by Joshua Noble, which has some interesting projects quite comprehensively explained, although more on the software than hardware side.
And I see in Amazon's "pre-order" category a forthcoming book by Jonathan Oxer entitled Practical Arduino: Cool Projects for Open Source Hardware.
I find most of the project-oriented books useful for getting exposed to topics and ideas and concepts even if I don't intend to actually make them. This is helpful to indicate what you might need to learn, even if they don't explain all of that.