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.