Which output style? I suspect the Author and trailing comma need to be isolated between two pipe characters in the template for book, to associate the comma with the field.
Also there are fancy handling options by APA criteria which may help (in anonymous settings of the output style).
Okay, I played with it and not sure what I did right, but see if this works now to expectations. Check both an authors and anonymous book, and you may want to check long and short forms of the footnote and the Bibliography version, which I played with too.