Children's thriller/adventure series surrounded around a group of early teen friends (fiction of course)

The series is centered around a group of friends going through their town solving weird mysteries/occurrences. It's a hard cover series and each book is relatively a quick read. (Potentially somewhere between 70 to a 100 pages-ish) Each book features mostly black and white illustrations on the cover, with sometimes a tiny splash of color. The best reference to its art style I can think of is similar to the cover of "The Boy With The Cuckoo Clock Heart" by Mathias Malziue. I read part of the series back in elementary school, around when I was 9 or 10. From that time, I can recall two books. One centered around a ghost in the library, and the other having something mysterious going on in the orchard. The orchard later goes up in flames towards the end of the book.