Gathering “Authors” In my stored procedure, how?

My stored procedure for monthly sales is now close to finished. The only thing left is that it prints out the "Authors" on several lines. So if a ID has several authors, it will print several lines instead of one. Can i resolve this in a Group By statement, or something else? And where should i put it?

How it is shortly showed(not with all the other stuff):

• 18.01.2014, Author 1, ItemID1
• 18.01.2014, Author 3, ItemID1

How it should be:

• 18.01.2014, Author 1, Author 3, ItemID1

  ALTERPROCEDURE[dbo].[LOID]@month         INT,@year          INT,@report_source NVARCHAR(255),@is_primary    INT ASSELECT Cast(isa.sales_date AS DATE)AS
             DATO,
             BV.name                                                        AS
             BU,
             isa.identifiers                                                ASIS,
             BB.name                                                        AS
             Fo,
             bc.name                                                        AS
             AUTHOR,
             bk.title                                                       AS
             Tizx,
             isa.quantity,
             Isnull(id.sales_price, Isnull(u.sales_price, isa.sales_price))AS
             SALES_PRICE FROM   book_sales AS isa INNERJOIN store AS BV ON bv.store_id = isa.store_id LEFTOUTERJOIN discount AS id ON id.identifiers = isa.identifiers AND id.store = BV.name AND id.from_date <= isa.sales_date AND id.to_date >= isa.sales_date AND id.to_date <'2999-01-01'LEFTOUTERJOIN discount AS u ON u.identifiers = isa.identifiers AND u.to_date ='2999-01-01'AND BV.name = u.store LEFTOUTERJOIN book_contributor AS BC ON BC.book_id = isa.book_id LEFTOUTERJOIN books AS BK ON BK.book_id = isa.book_id LEFTOUTERJOIN publisher AS BB ON bb.publisher_id = bk.publisher_id WHERE  Month(isa.sales_date)=@month AND Year(isa.sales_date)=@year AND isa.report_source =@report_source AND bc.is_primary =@is_primary