The Book is a book on magic maintained and studied by the Carpenter children. It is mentioned in "Zoo Day".
The Book was started by Molly Carpenter, before she grew up and forgot all things magical pertaining to childhood. Her siblings added to it in turn.
In the series
In "Zoo Day", Harry Carpenter had passed the book to Margaret Angelica Dresden, and Mouse read it with her. It specifically references haunts, their characteristics and the ways to deal with them.