I'm against ads in novels, except maybe discreet notices about similar titles, placed in the end pages. A German publisher once inserted a text ad in the middle of one of my books, using the characters from the story and saying it was time for them to take a break and have a warming cup of XXXX soup! The first I knew about it was when the UK publisher sent me the complimentary copies. The UK publisher didn't want to take up the cudgels on my behalf, so I hired a lawyer and got the German edition pulped (they were in breach of contract), plus a bit of compensation. Then we sold the German language rights to another firm.
Novels are a special case. You don't want to have the world of the book shattered by some intrusive message. Nonfiction might be different. As nonfiction ebooks become more sophisticated, with hypertext, the temptation for publishers will be to include sponsored links. This should be resisted!