mysql, view, ERROR 1242 (21000): Subquery returns more than 1 row

Hi can someone help me with this please,

i have a view where i am joining up several tables so is easier to query, i seem to be having this error stating subquery returns more than 1 row.

create view full_book_view as select b.book_id, b.book_title,b.isbn,b.retail_price, b.new_price, b.postage_price, b.description, b.quantity, b.book_condition
			    , (select group_concat(a.author) FROM book_authors as ba 
			    inner join author as a on ba.author_id = a.author_id
			    WHERE ba.book_id = b.book_id
			   GROUP BY ba.book_id) as Author,
			   (select group_concat(pub.publisher) from book_publishers as pb
			   inner join publisher as pub on pb.publisher_id = pub.publisher_id
			   where pb.book_id = b.book_id group by pb.book_id) as Publisher,
			   (select url from book_url as bu where bu.book_id = b.book_id)as url ,(select cust.username from customer as cust inner join users_per_book as upb on upb.customer_id = cust.customer_id where upb.book_id = b.book_id) as customer
			from books as b
			inner join book_categories as bc on bc.book_id = b.book_id
			inner join categories as cat on cat.cat_id = bc.category_id;

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thanks