More depth on the actual mechanics of windmills than anywhere else currently published in English. The chapter on Sails is the only treatment I've found, and explains the different windmill sail types quite understandably. The governor mechanisms introducted to smooth out windgusts into smoother power sources are also very interesting, while not bogging down the text. For those interested in the math and equations, the biography lists extensive historical sources.
Very good coverage of British and Dutch windmill technology. American style windmills, only used to pump water, are sparsely covered --"A Field Guide to American Windmills" is not in danger of being challenged here.
Windmills being used to grind grain or pump water are of course well covered, but the harnessing of wind for other industrial uses such as sawing in lumbermills, pulping paper, and uses in wool industry are also explained in their own chapters. Good to see these lessor subjects expanded on.
The author does spend some time reiterating the standard arguments for where vertical windmills *didn't* originate historically, but holds back from stating the growing if still controversial view of Viking origin for this invention sometime between 700-1000 AD. He simply states the known case of the earliest recorded windmills existing in Europe around 1300 AD, and Persians recording the first windmills they'd seen being built by the Crusaders and then spreading the technology East, and only in crude form. Hopefully future archeology digs will settle this question.
Other historical technology books focus on recording specific historical windmills and their sites. This book gives the overview on how construction and mechanical methods improved, how those improvements spread (not uniformly!), and which proved best in actually getting the job done. While most windmills must by necessity remain with their sails stripped and idle as part of preservation, this book gives a detailed view of how they actually worked and moved in their time.