No wonder the book's tagged that price; come to think of it , it might be a reasonable price. Denis Diderot was one of the finest minds in france in the 1700's. Like the person told you over the phone, this book is one of the volumes of his ' Encyclopedie '. He's basically singlehandedly responsible for the form of the modern encyclopedia.
The books text isn't going to do us any good, at least it isn't going to do me any good as I can't read French but the engraved and/or etched plates of artillery certainly would be fascinating to look at.
This book may never belong to any of us but it's content already does. 75 years is the longest possible term of copyright in the U.S., so everything published before 1931 is concidered to be in the public domain. If you have the energy and inclination, I still think you'd have the greatest chance of finding a copy at the N.Y.C. Library. I'll bet that right about now your starting to regret that stroll down Lexington. My best bet would be the main branch of The Chicago Public Library, Im going to call and check it out.
John