What is the significance of the title, The Book of Lost Things?

The Book of Lost Things is the title of this book, but it is also the title of the book held by the king of Elsewhere. David believes the king uses the book for guidance in times of trouble, and the Woodsman suggests the king might use it to help David get home. When David finds that book, he discovers that it's actually Jonathan Tulvey's journal, and that it represents all the things he lost when he fell for the Crooked Man's trick.