Reckless by Cornelia Funke is certainly my favorite series and has given me a ton of inspiration for my own writing throughout the years. I've read this particular one a few times, one of those times being recently. The story itself appeals to readers who enjoy fantasy and medieval-based times. There is little modern technology except in dwarven cities. It gives a taste of the world we know compared to the one we explore in through flashbacks or character background, but the focus is on Jacob Reckless as he struggles to keep those two worlds separate. Initially, he goes through the mirror in search of his father who he accuses of abandoning him and his younger brother Will, but later it reveals that the Mirrorworld has taken on a life of its own. Will travels through the mirror by accident and ends up getting infected by a Goyl, which slowly begins to turn him into the prophet jade goyl they have been waiting for. Jacob, with his companion Fox, are faced with the predicament and handed the task to try to save Will's life before it is too late.
It will forever be one of my favorite stories with the way it is so well written. It's a story that has you holding your breath, hoping that the next act will go in the protagonist's favor. However, it doesn't lead to an easy happy-ending, and leaves with a giant twist at the end of the story.