In the late 70's my elementary school library had, as best I can recall, a series of smallish, library-bound books that were a series of spooky stories for kids. They were quite eerie and had black and white illustrations that were similar to the kind in Scary Stories to Tell in the Dark, but this was well before that book was published. It is also not Horror Tales ed. Elwood. In terms of the date they seemed pretty new circa 1978-79.
The one story I remember was about a girl who was friends with another girl who happened to be a ghost.