The Witch Who Was Afraid of Witches by Alice Low, illustrated by Karen Gundersheimer (1978).
This was one of my Halloween favorites growing up. It's the story of a young witch who lives with her older sisters who are bossy and mean to her. They make her feel like she's not good enough to do magic like they can. But one Halloween she befriends a trick-or-treater and learns she really does have witch magic.
It was reissued in 2000 with new illustrations by Jane Manning and made into an "I Can Read" book. From the images I've seen of that version (Amazon), they are perfectly nice illustrations but lack the quirkiness of the original version. It's actually really interesting to compare the two to show how a story can have such a different feeling depending on the illustrations. The expressions of the character faces in Gundersheimer's version just can't be beat.
See my Pinterest board for more Halloween books.