Where do you add series data in the v3 book editor?

In the v3 book editor (both the sidebar and full-page views) I couldn't find any way to add a series to a book. The fields for title, subtitle and author are obvious but there's nothing labelled series anywhere.

Resolution: Series is added with the unlabelled centered + button that sits between the subtitle field and the Author field in the book editor - clicking it opens the series name and number inputs. This was purely a discoverability problem: the control worked, it just had no label, which is why several people found it once and then couldn't find it again. A label was added in a later v3 beta, so on current builds the control identifies itself.