The Patronus spellbook is a book with instructions and information on the Patronus Charm, owned by Remus Lupin. Lupin used it during one of his Anti-Dementor lessons with Harry Potter. He placed the spellbook within a secret passageway accessed through the cupboard in Professor Cuthbert Binns's History of Magic classroom, and put a Boggart there also, knowing it would become a Dementor. Harry took the book with him and presumably gave it back to Lupin afterwards.