Lately I've been wanting to learn more about interior design theory so I've been sourcing old design books for myself as learning tools. One book I recently purchased was Mark Hampton on Decorating. In this book, the late and esteemed decorator Mark Hampton (who David Hicks' partner before starting his own firm) coves various aspects of design such as color, the use of accessories like draperies and rugs, room layouts and much more. He also shares how readers can incorporate his design principles into their own homes.
Although I haven't yet finished the book I am thoroughly enjoying reading it and find it very inspiring. It's definitely a book worth checking out. Another out-of-print design book I recently purchased is Carleton Varney's Decorating with Color. Renowned interior designer Carleton Varney was a protege of Dorothy Draper and has been the president of the Dorothy Draper Company since he was 29, a position he still holds today. Varney's design style embraces the use of bright colors, bold patterns and floral prints.
If you need some direction on where to shop for out-of-print books, I always have great luck on ebay or on Barnes & Nobles' "Used & Out of Print" area of their website. If you've never read these two titles go find yourself a copy now!