This book is wonderful. The world Tove Jansson created is a complete one-off, an enchanted and enchanting place for children to escape to in their imaginations.
According to the publishers, this book is aimed at readers 9+ in this translation, but really I wouldn't hold your child back if they are ready for it. My seven year old son is currently reading it, and while I'd say it's certainly the most difficult book he's read so far, he is lapping it up. Every time he puts it down after a stint of reading he cmes to tell me what has been going on in Moomin valley.
In addition to the exciting story, the book contains many illustrations of the many weird and wonderful characters, and these serve to bring the story even further to life. My own childhood experience of the Moomins was that slightly weird but oddly charming Polish animated seires that ITV put on the in the early 80s. That was good, but you can't beat the real deal, and I'm so glad my son is reading the books for himself.
Also worth mentioning is the fact that this book comes in a very nicely made hardback format. It seems you pay a little more to get this particular edition, but I'd say it's well worth it. Not only does it mean it'll last longer (I have two other children who doubtless will get round to reading this book) but the hardback is really well made, giving it a classic feel which matches the contents.