The premise is simple: Hockney believes that the Old Masters of European art used tools and techniques of which little record remains. This book presents his justification for that belief.
The first half of this book is visual. It shows the original paintings and drawings that led Hockney to this idea. Once it's pointed out, many signs are unmistakable: odd proportions in otherwise masterful works, inconsistent perspective drawn by people who really knew perspective, and a few other better-known oddities. Although I'm not a fan of Hockney's own work, I respect the training and sensitivity that picked out these features.
Hockney goes on to show how these artifacts could have come from use of a family of optical tools, including camera lucida and several variants on the camera obscura. This is where he brings the most to this book, in trying the tools himself, as an artist, and seeing what unique features each tool imposes on the resulting artworks. This is what has so many critics upset - the idea that the Old Masters might have used every tool possible to complete their commissions faster, and to give their patrons the most pleasing result for the ducat. Those critics know about the assembly-line work in some of the Old Masters' studios and who know about the other mechanical aids that are well documented, but squawk at the idea of adding another tool to their toolboxes. Huh?
Hockney's evidence is often circumstantial, since painting was (and often is) a secretive and competitive business. Still, he offers a good story, and the second half of the book adds a strong foundation of written records to the structure. This is the book's weakness, though. Hockney is an artist, not a historian or optical technologist. He chose a story-telling format for presenting his findings, the letters he exchanged with scholars and specialists in other fields. It has a friendly look, but lacks in density and in organization of the historical records.
Despite its many flaws, I find it a fascinating study. Hockney really brings history to life, with his own hands, dispelling the idea that historical study is a dry, dusty practice. His documentation lacks in formal rigor, and he addresses the Great Masters about whom people have strong sentiment. Some people see that as iconoclasm for its own sake - guys, get over it.
-- Address his facts with facts. Name-calling says more about you than him.
-- Picking one nit (and there are lots) doesn't pick apart the whole presentation.
-- Don't assume that Hockney's own art (of which I'm not a fan) decides the merit of his historical analysis.
-- Accept the idea that his eye may be better than the words he can put to his vision.
It's an honest and vivid account, with a good base in reason and fact. It deserves respect on that account, and works hard to earn the reader's enjoyment. I recommend this to anyone interest in the history and practice of visual art.