I recommend Encyclopedia Brown. I was 8 when I first encountered this series. I loved it. It was recommended by a boy that I liked. What I liked about these books is that it taught me how to think before making wild guesses. Each book had several cases which were easy to read and follow for a child. I recommend this to any parent looking for helping their child to use logic and deductive reasoning. Great on how to look at relevant facts at a child's level.