iBooks help - series

As far as I know, books are listed together as a series when they are part of one. However, there are some books that are part of a series that are not categorised together. How do I force the books to be categorised as a series? For instance, I have the first four books of the John Milton series together. Then I have the series of Ancient Legends by Jayde Scott that are not categorgised as a series, but listed individually. I had to resort to creating a temporary fix by creating a collection, which doesn't sit well with me when I see the other books I have. I'm a bit OCD, I guess.