> It's just that the ads are not wedged in between the middle of the book to interrupt the reading experience.

Absolutely, the annoyance-factor of some types of advertisement are within acceptable bounds. It also helps that those ads may very well be for things you're actually interested in; if you liked the book you just read, knowing what other books are available from the same author at the end actually helps you. This is in stark contrast to, for example, the ads at the end of articles on some pages that masquerade as other articles, but just take you to another page filled with more ads for things you have no interest in.