I don't see a problem with it as long as you aren't telling them how to do their job, what they need to do to make it better, or how things should have gone in their story, etc. Which it doesn't sound like you have intentions of doing. Most authors have a mailing address or email listed somewhere in their online bio these days. There are a lot that are even available for direct communication through various messing apps. A lot of them are more easily accessible due to social media. Some even have author pages and fan groups that they're active with. If they're bigger name authors you're less likely to get a response though. But from what I've seen, most authors love to get messages like that. It makes them feel good to know that their work has had such a positive impact on people.