Decline from what would be a better measurement I think, personally I think Europe probably reached it's peak at about 2000 and has been steadily circling the drain since.It looks like a good book actually (I just bought the audible version, so will give it a go). Like all books though, and history in general, it will present an incomplete story of Europe and the reasons why Europe is as it is today.
When talking about the 'decline' of Europe, what are you talking about exactly? Decline from when? It's more peaceful in Europe now than in previous times and Europe is thriving economically with the EU as a whole being the second wealthiest and second largest economy in the world - so from which point has it declined?
Peace is a bad measurement since war is only the visible and generally final stage of conflict.