This list includes series I've read fully even some with only two books. Any Manga series I completed are represented by just one square because some of them have 20 plus volumes. This also includes in-progress series that may have other books released at a later date (I'm looking at the Montague Siblings series and Creekwood).
This list also stretches back to my childhood. The American Girl books listed are the girls where I read all six of their original stories. I've read at least one book about all the American Girls before 2006 but I don't think I read the whole series of any of them except Kristen, Felicity, and Molly.
There are a lot of book series where I have only read the first book or partially read the series but I didn't want to include those.