While a book can only have 1 series, it can have multiple tags. This is really handy as well.
So you have your series 'Jax' and each book numbered 1,2,3. You can also add additional tags that you can view in your collection view. For example, you could tag books as 'Sci-fi' or 'Favorites' or 'February Reads'.
I use it to collect series together. For example, books by different authors and different series but under the same shared world. Comes in very handy.