. What if any door were every door? A compelling story about a world mostly like ours except there is a set of magical books that have special powers for those who have them. The Book of Pain, the Book of Joy, the Book of Memories and so forth. Cassie gets given the Book of Doors and discovers that there is a huge shady underworld of people who want these books and will spare no expense to get them. And of course there is a hidden library. There are some pretty evil evil people and that was a difficult part of this, for me. Just the right amount of sentiment and an interesting story.