I didn't even know The Moomins existed until a few months ago, I know it's for children but I've been really enjoying watching the show, and now I'm tempted to read the books at 24 years old especially because I have a LOT of questions about the story, like first of all how come Snufkin doesn't even know Joxter and why wasn't Joxter around to raise Snufkin and what not ?!? I'm judging you Joxter
Don't let the idea "it's for children" throw you off. I'm a generation older than you and I just re-read several of the Moomin books. They are profound and thoughtful, like many of the best "children's" books ("The Wind in the Willows," the Pippi Longstocking stories, "The Little Prince") as well as funny, clever, and touching without being sentimental.
My favorites are "Tales from Moominvalley" ("The Hemulen Who Loved Silence" taught me, a talkative extrovert, a lot about introverts) and "Moominland Midwinter," which has a lot to say about adversity, resilience, and self-discovery. But you probably want to start with the earlier ones, like "Finn Family Moomintroll."
Happy reading!