Normally I'd just link to an aritcle like this ,but I've already seen this reported as fact. The following is a joke, but the satire is subtle. I don't want you to think I'm taking this seriously.
There's an article in today's Seattle PI about the first in-book advertisements:
What is little known is just how far back this practice dates. Recently, the Annenberg Center for Communication, established to cleanse the family name of patriarch Moe Annenberg's highly dubious activities, has been conducting a survey of nineteenth century British literature. To their surprise, they have, in the process, discovered advertising so subtly placed within classic texts that it has hitherto gone unnoticed by scholars and readers alike. Many of the ads, it has since been learned, were part of an ongoing campaign by Ogilvy & Mather, the ad agency originally established by Patrick Ogilvy and Cotton Mather in the 17th century, with offices in Edinburgh and Boston, to promote fire, brimstone, and treacle for everyday use in the home.
...David Copperfield by Charles Dickens
I Have a ChangeThe carrier's horse was the laziest horse in the world, I should hope, and shuffled along, with his head down, as if he liked to keep people waiting to whom the packages were directed. I fancied, indeed, that he sometimes chuckled audibly over this reflection, but the carrier said he was only troubled with a cough. If only he'd given the horse Dr. Locock's Pulmonic Wafers. They provide perfect freedom from coughs within ten minutes and instant relief and a rapid cure of asthma and consumption, coughs, colds, and all disorders of the breath and lungs. The carrier had a way of keeping his head down, like his horse, and of drooping sleepily forward as he drove, with one of his arms on each of his knees. I say 'drove', but it struck me that the cart would have gone to Yarmouth quite as well without him, for the horse did all that; and as to conversation, he had no idea of it but whistling.
You can read the rest of their spoofed examples here.