Image above, 1971 rare edition of Jansen Decoration, Mecox Gardens.
Currently featured on 1stdibs is a great illustrated article, Maison Jansen: A Century of Design, by Louise Devenish of Parsons. If you want to learn more about the legendary interior design firm, her article offers an excellent (and free) crash course. Related books tend to be pricey: last year's Jansen (20th Century Decorators) retails for around $90, and every copy I spot of the 1971 Jansen: Decoration, seems to cost $2,200 or more. (Yes, thousands.)