One of my favorite things about this series is the vocabulary is readable on a third grade level, and includes plenty of pictures to support the text. Another great thing is that readers don't need to start with the first book to understand the characters or plot of each book. Each volume reintroduces them, but not in a roll-your-eyes-because-you've-read-it-before kind of way.
I'm always looking for great series books that reluctant readers can sink their teeth into. Do you have any that you'd like to share?