One of my favorites growing up was called the A to Z Mysteries, by Ron Roy. The Absent Author, The Bald Bandit, The Canary Caper, etc...those really improved my retention ability. Another one was The Boxcar Children mysteries. Four children are orphaned, and live in an abandoned boxcar because they believe that their grandfather is a cruel old man. After one of the kids falls sick, they seek help from a doctor, which leads to a chance meeting with their estranged relative. As I recall, there were at least 200 books in that series, not including "Special Edition" stories. But then I stopped reading after I found out that the author, Gertrude Warner, had passed away after writing Book #19. Every book published after that was penned by a "collective" of writers using her name.