Often presented as the first Moomin book, Comet is a little bit darker than the others. It does, after all, concern the coming of a comet! Each of the odd personalities of Moominland copes with this knowledge in a different way-- the story becoming more about how they see the problem than the problem itself. Because of this focus, it is the perfect introduction to the personalities presented in the rest of the series.
Some parents may be reluctant to share this book with their children because of the darker themes (and highly improbable mathematics), but I think it would be a fine choice for a prolific reader in the fourth grade or higher.