This is part of the Family Moomintroll
series, written by Finland-born Tove Jansson
(Who sadly died on June 27, 2001). This is also probably my favorite book of all times.
As the gather from previous books, all good MoominTroll stuff their tummies with pine needles and go to sleep for the winter, however, something unheard of has happened, little Mummin wakens and can't go back to sleep. He leaves his Moominhouse to explore this mystical, dark world of winter that no other Moomintroll has seen before. This is also the book in which we meet one of my favorite characters, Tuu-Tikky, who lives in the bath-house, and we see a lot of Little Mue and some Morra.
I am not going to get inot why this book is so special, cause to qoute Tuu-Tikky, "You have to learn it on your own".
But in my honest opinion, it is the most serious of all the Moomin books.