I don't remember what grade I was in when I started reading the Boxcar Children Mysteries. All I remember is reading Volume One in school and having my mom purchase volume after volume, because I absolutely Loved these stories.
Another series of books that I really liked was "The Babysitter's Club." I think I read up to volume 33 before losing interest. There was a Babysitters Club movie released. I didn't really like the movie, but I did want to start my own Babysitter's Club. That never happened, but the books were a joy to read anyway.