Series of children’s books with supernatural storylines

Question

I have been trying for years to remember the name of this series of books that I remember from my primary school in the late 90s.

They were a series of books that were designed to develop reading skills in young learners. I feel that they were written in the 60s or 70s perhaps. The books and illustrations had quite a green/greyish tint to them. The stories often had a spooky or supernatural story to them. I remember one book was called Something like Will o the Wisp. Something tells me main protagonist may have been a boy called Caspar.

Answer 1

Aside from the dates, this sounds a lot like R.L. Stine's Goosebumps books. There were many of these, starting in 1992, and the covers often incorporated green and purple color schemes. The protagonist characters were children who wound up in scary circumstances.

Over time, there were a number of different Goosebumps series, totaling tens of books, and overall, they are the second best selling book series in history, right behind the Harry Potter books. There were also television adaptations, the first one with R.L. Stine himself, then later ones with Jack Black playing the author.