There was a recent piece in the WSJ about advertisements in ebooks.
There are mentions of ads on the margins of ebooks and ads at the beginning, middle, and end. Clearly some of these ideas could be disruptive to an immersive reading experience.
There is an idea for advertisements in ebooks that I wish would take off because I would like cheaper ebooks. That idea is sponsorship for ebooks. Take a company that would like to advertise. The Acme Widget Company for example. You go on Amazon to buy an ebook and you see two choices for the ebook. One version is the standard Kindle version and it cost $9.99. There is also an Acme Widget Company version for $5.99. You purchase the Acme version of the book. On your ereader the cover page of the ebook says that it is the Acme Widget version of the book and after the cover there are a few pages explaining some Acme Widget products.
If readers would get a direct price reduction when purchasing an ebook with advertising I don't think many people would object. And if someone does object they can always buy the $9.99 version that has no advertising. The strongest thing about this idea is the goodwill that a company would earn by saving someone money on a book they wanted. I think that some companies would go with the tact of only mentioning their sponsorship on the cover. The act of saving book buyers several dollars at the time of purchase would give many people a positive outlook on the company that just provided the discount for the book they wanted.