This article on Engadget talks about Microsoft’s patent to put ads in Ebooks:
We have ad-supported e-reading today, but the ads always sit on the periphery at most. That makes us more than slightly nervous about a newly-granted Microsoft patent for contextual e-book ads. The development would make the pitch based on not just targeted pages but the nature of the book in question: a sci-fi novel might try to sell lightsabers, and characters themselves might slip into the ads themselves if there’s a fit. Promos could be either generated on the spot or remain static.
What it doesn’t say is if these ads will appear in free books or books that we pay for. If I have to stop every few pages while reading an ebook that I paid for then I’m going back to print books only!