What were the books where a brother and sister would solve mysteries and the answers were written in reverse?

I used to read these books all the time. It came with a picture that the brother would have drawn of the crime scene. The sister would interview people and by the end of the story, they'd have solved the mystery. Then the answer would be written in the back of the book, written backwards. You could either learn how to read the story backwards or look at it in a mirror and read it that way.

Does this strike a memory with anyone?