I love Gaiman's works, Corilane was one of my favorite children's books. I had high hopes for Mirrormask, we own the movie, and though it's a bit dark for the younger children, it's still a very creative and visually stunning film.
After reading the book I'm glad that the movie was visually stunning. For some reason the book does not flow, or hold together well. Had I not seen the movie I would be completely lost in attempting to follow this book. The concept of standing on the books to cross the gorge, or the giants locked together, or the darkness and what it is doing - is all lost. Now you can say that it was for the simplicity of making it a children's book - but I don't buy it. The concepts involve possible death, a brain tumor, and an evil girl willing to sacrifice the entire world she lives in to try something new (not our main character - the bad guy).
The book follows a young girl who has grown up in a circus, she want's to live a normal life and gets in a argument with her mother about it. Her mother then collapses and is hospitalized with a brain tumor. Feeling quilty, Helena withdraws into a world of her strange artwork. That night she wakes up to find herself drawn into a world of her own creation - someone has stolen the mirrormask and traded places with her. Now she has to travel through a strange and hostile world ruled by a Queen of Light and a Queen of Darkness, to find the only way home - the stolen mirrormask.
The artwork was good and the concept is great - but to read it without having seen the movie will leave the reader a bit perplexed. If reading to children, go with one of Gaiman's other works before this one.