Authors putting up ads on answers.

Yes, these authors want to be paid for their ideas.

I have not seen an example of this yet but think that as long as the book is relevant to the question i.e. "Can you share puppy-training ideas that have worked for you?" and they are offering the sale of a puppy training guide then it's okay.

Otherwise, I think it's rude and I'm pretty sure that if it weren't relevant (maybe even if it is) then it would be considered spam.

It costs money to keep these sites up and running.  From what I've seen, this site is a Google site and there are only a few advertisers allowed to post their ads.  One is Amazon.  I don't find this too intrusive.  There could be Verizon wireless ads and other irritating commercial crap.  At least Amazon tries to post books for sale that address the topic at hand.  Either way, the money to keep this site going has to come from somewhere and that means advertisements.