The Chestnut King

The Chestnut King by N.D. Wilson

When Henry York found 99 cupboards hidden behind his bedroom wall, he never dreamed they were doors to entirely new worlds. Exploring them with his cousin Henrietta was all the action and adventure an overprotected boy could imagine. But Henry's discovery also unleashed the undying witch, Nimiane, who's hunger for power could destroy every world connected to the cupboards - and every person whom Henry loves. Now Henry must seeks out the legendary Chestnut King to answer the unanswerable. How do you kill someone who cannot die?

Here's the book trailer.