Reckless by Cornelia Funke
Jacob Reckless is 12 years old when he discovers the secret in his father's abandoned study: a mirror that can transport him into a magical world of fairies, witches, dwarves and shapeshifters. He spends twelve years exploring and exploiting the magic, and thinks less and less often of his sickly mother and lonely younger brother...until the day that younger brother Will finds the mirror portal and immediately falls into trouble in the Mirrorworld.
German author Cornelia Funke's skillful blend of traditional magic tropes (child-eating witches, enchanted apples, princesses spelled to sleep until kissed awake) and fantastically horrible original creatures fills the quest to save Will from certain doom with a dark (very dark) charm.
This series was originally cataloged and shelved with the children's collection at KCLS; on review, the series will be shifted to the teen collection due to adult characters, somber themes, blood and violence, and references to sexual situations in later volumes. It is much darker and more violent than this author's Inkheart series...and it is possibly a stronger story because of the darkness.
Highly recommended for ages 12 to adult.