The series is made up of 6 books.
The plot of this series is one of mystery, by genre you might call it a spy novel. It is great since it has the same depth as an adult novel, but is written for preteens and teens.
Plot is great, but this story wouldn't have gone so well without the amazing cast of characters
Both those that stand out on their own and those that fit more as part of a larger group.
Some people fit into multiple of these groups, but they will go into the group that fits them most, and people that belong to a group but get their own page might be referenced, but not talked about a lot there.
There is many things to think about, and while plot and characters are big, there is also setting. Setting, something set up in the title of all of the books. So learn about the six places mentioned in the book titles.