I purchased this book on sale at the Kindle store to get a detailed sense of what his methodology was, and it seems he just uses Google Trends to compare search volumes across locations combined with Google AdWords to give the data a numerical scale and break down the data demographically. It seems statistically dubious at best, especially the part where he makes assumptions based on search autocomplete results (which vary based on location, IP address, browser, OS, and other factors).
You can read more about his methodology in his papers here [1]. He wrote his PhD thesis on "Essays Using Google Data". I can't think of a better way to do these estimates with the limited data provided, but I'm just not convinced it's a strong enough proxy for the conclusions he's making.