I read just as I was beginning to try and get to terms with the latest Biblical research in this area. This is a great place to start before getting into more hefty works by the likes of N T Wright. As Grove books say - not the last word but often the first.
I found this book has a very useful summary of Tom Wright and it greatly helps to fit all the pieces together. It will be an invaluable reference for a quick memory prompt. It's also very motivating in showing how the theology works out in practice. Also encouraging is the fact that Stephen does not agree with Tom Wright on every detail. The style is also uplifting - full of grace and truth.