This kind of advertising is really old. I have a 1930s Hardy Boys novel that references an earlier book in the story, and contains mail order offer in the foot

This kind of advertising is really old. I have a 1930s Hardy Boys novel that references an earlier book in the story, and contains mail order offer in the footer ("Send your order plus 20¢ to ____ to get your copy of ____. Allow 10 weeks for delivery" - definitely not Amazon).