The Maltese Clue

From the front cover:

"You, Same "The Spade" Lonetree and your band of expediters are given the task of locating the key to a magical book for a powerful Wizard. The key - of which he can give only a vague description - is located in a castle that to all appearances is haunted. Strange things do happen, and your employer informs you that two previous parties have failed to return. Muttering something about talking artifacts and black birds, the Mage says, "I have no choice; it's a job for The Spade. Good luck.""