This book is an abridged version of the author's much larger full biography, Never at Rest, published in 1980. In the preface the author notes that this is very much an abridgement rather than a rewrite, because his interests have changed and he could not do justice to the research on Newton that had gone on in the intervening years to 1993. This makes the work 28 years out of date, although in historical terms it is hard to know if this is an issue.
The perceptible difference is in the tone. Early on in the book, especially, Westfall adopts an almost sycophantic attitude to Newton's genius, constantly assessing each event in terms of whether it demonstrates the great man's ability, and nudging us knowingly when other thinkers have not recognised this towering intellect before he has published anything. I'm not sure a (non-revisionist) writer setting out to write Newton's biography today would adopt such an attitude, and would hopefully be more inclined to tell the story and let the events speak for themselves.
Happily, as the book goes on and Newton's talent is recognised we are given a glimpse of the man himself, and it truly is a fascinating vision. I found Newton's obsession with alchemy and the Holy Trinity (which, for him, represented the Beast of Revelation) even more fascinating than his work on optics and gravitation. Newton comes across as a man of almost aspergic obsessiveness and aversion to engage in normal social interactions, one who set terrifyingly high standards, both intellectual and moral, which only he, working prodigiously in a position many treated as a sinecure, could ever hope to aspire to.
I can see why the author was asked to produce this book: it is about the right length for someone peripherally interested in Newton, and without the mathematics, which many would find off-putting. In the 20th century Einstein is generally portrayed as the quintessential genius, tousle-haired and absent-minded, but anyone reading this biography of obsession and single-mindedness would be forced to pass the mantle to Newton.