Your book can be part of a series or collection. To be able to add it as metadata to your book, you first need to create the collection/series.
/!\ NOTE: When selecting the series or collection your book is in, you need to add the Order (rank of the specific volume) so that online bookstore will include the book in the series. Without the order, your book may be published as a separated entity.
Please be aware that metadata for series is not currently compatible with Google. They are working on the feature but don't yet know when it will be ready.
Here is how to add a series: