Karen's Witch: a Graphic Novel (Baby-Sitters Little Sister #1) - By Katy Farina & Ann M. Martin

Karen Brewer lives next door to Mrs. Porter, who wears long black robes and has wild gray hair. Mrs. Porter has a black cat named Midnight and always seems to be working in her garden. Karen isn't supposed to spy on her neighbor, but she's determined to prove that Mrs. Porter is a witch named Morbidda Destiny