This book seems a re-hashed attempt to create an unnecessary buzzword / brand to present ideas that have previously been put forward in other, more thorough research/discussion. His term 'Globish' is an arm chair journey, with anecdotal accounts of English spread and no reference to any research or opinion from the many fields that discuss English in global contexts.
I find it amazing that this book has received so much attention, and that McCrum now writes as a popular oracle on English language spread, when, in fact, his omission of previous literature on English as a Lingua Franca, Global Englises, World Englishes, English as an International Language, English in a Global Context, Language Spread and New Literacies ought to qualify him to be one of the last people to be asked about related areas. What is the most striking feature of this book is certainly the lack of empiricism, and the side-step he has given those scholars who expressed similar views (far more effectively and with more relevance, I might add).
In short, it is hard to see this as 'research' on English around the world, it is not well argued (partially as a result) and I believe it is unethical, as he used some phrases in an interview with Andrew Marr which were clearly not his own, and which in turn suggested very clearly that he had read and adapted previous thoughts and research for his book with absolutely no reference to them. Also see Michael Swan's review of this book, which points out its many weaknesses in a more detailed but refined way.