Many authors have a "Contact me" page on their website. Check there for the best email to send fan mail to (or, sometimes, perhaps a posted letter). Many bigger authors will mute their notifications on social media, so tagging them there may not get noticed, but it can be an option, especially for newer/smaller authors...

I will say that as long as you 're reaching out to say how much you've enjoyed something, it will be appreciated. If you're reaching out to let someone know about a spelling error or how you think the story should have ended or anything like that, just don't...but genuine fan letters are welcomed. And from everything I've heard from authors, and other artists, it happens less often than you might think! Of course, a small handful of people get overwhelming attention, but the majority of authors aren't in that club...

I've personally written thank you notes to authors, musicians, actors, directors, set designers, and others through the years. I don't always get a reply, and that's fine. But I often do, even if it's probably a form response. And a few people I've connected with to the point that we regularly stay in touch.

So definitely go for it. All you're doing is putting a little kindness into the world, and that is a good thing!