Read this if you read Elixir. It is similar, people flying around the world at the drop of a hat and acting like spies but this book has more depth to it than Elixir. There's more philosophical struggle and more character struggle than the first. Also, Duff does a really nice job of constantly driving the plot forward, never resting on one thing too much.
Fast paced, exciting and creative. There is the introduction of characters who are not "real" that I really liked because it added an extra dimension (literally) that layers the book to an extent that was missing a little bit in first book.