You should be aware that you probably don\’t have a duty to inform anybody about a suspected infringement. However, publishers and authors (and especially the smaller businesses) often appreciate this information.
If you tell anyone, it should be the copyright owner. Don\’t make any spurious allegations; simply contact the copyright owner (or the person you think is most likely the copyright owner, such as the publisher or author) and tell them what you saw, and where, giving as many factual details as possible. Let the copyright owner decide whether it\’s infringement or not. You may phrase your information like an inquiry: for example, \”I saw such-and-such at this location. I was wondering, was that a legitimate copy/licensed by you/being done with permission?\”
It is not wise to go public with a mere suspicion of infringement. It is possible that a statement like \”so-and-so is infringing copyright\” may be actionable, in particular if it turns out not to be true.