Pure Dead Trouble, Debi Gliori

The Strega-Borgia family lives in an old castle in Scotland. They have an alligator in the moat, Beasts in the dungeon, a witch as nanny to the children, a cook who can't cook and a butler, who they find passed out on the doorstep when they return from vacation.

Only the nanny realizes what happened to the butler, and she must investigate and plan on her own.

This book was a fun, fast read, and you probably don't need to read the three that came before to enjoy it.