Combine Authors With Similar IDs

Im trying this simple query, where if a book has the same ID but different names it should be shown in one row and not two...

Now it is showing

- 1 AuthorDude
- 1 AuthorGirl
- 2 Authorblabla

While it should show

- 1 AuthorDude, AuthorGirl
- 2 Authorblabla

SELECT a.book_id, b.Name
from BOOK_SALES a
left outer join
(
select Book_id,is_primary,
STUFF((select ', ' + Name from BOOK_CONTRIBUTOR B where B.BOOK_ID = A.BOOK_ID
for xml path('')),1,1,'') Name
from BOOK_CONTRIBUTOR A
group by Book_id,IS_PRIMARY
) b on a.BOOK_ID = b.BOOK_ID

When i do a inner join, it would just show many Book_ID`s, but with the authors combined.

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