This fantasy/thriller book could be mistaken for a children's book, if it was opened randomly, as some passages seem to be the retelling of fairy tales, but that is superficial. The stories are twisted from their "traditional" story lines.
It starts off fairly normally. David, a 12 year old boy is going through routines he has set himself that he thinks will help keep his Mum alive. His routines don't help, and she dies, as is expected. [After all, how many successful storylines are based around normal families with no problems?]
His father remarries, and David dislikes his new Mum and baby brother. He sees them as interlopers, and it is their fault that his family is not like it used to be, with usual childish logic.
He has also been having what sound like epileptic episodes, (although they are never named, neither is the cause of his Mum's death, other than it slowly ate her away.)
One day he is particularly nagry with his stepmother, and is told to stop in his room until the following morning. During the night, he goes out into the garden, and passes into another world.
It is in this world he meets the corupted fairy tale characters. As he travels to meet the king, in order, to consult "The Book of Lost Things", which he is told will help in get home.
He meets people who help him, and people who he has to overcome. As he meets each challenge, he starts to change from an impulsive child, to a more thoughful adolescent, well on the way to being adult.
It is a fascinating read, and has an ending similar, in my opinion to "Life of Pi" by Yann Martel
To find out a little more, here's a link to John Connolly's website, (to the page about this book.)
http://www.johnconnollybooks.com/novels_lost.html