The Book of lost things

This story is set in world war two, where the main character (David) stumbles into a world of mysterious creatures. David unexpectedly brings his fears into the world and creates monstrosities without knowing it. As the plot thickens, the crooked man appears. He seems kind, but is actually plotting David's doom. This story is so full of suspense, I couldn't put it down! It's a bit disgusting though.