Trigger that updates another table when inserting and deleting from one table
Something along the lines of the below... just join to the inserted/deleted tables.  Please do test this though with real life scenarios:

ALTER TRIGGER trgUpdatesNrOfCopies
ON bookcopy
AFTER INSERT, DELETE
AS
BEGIN
   SET NOCOUNT ON

   IF NOT EXISTS(SELECT*FROM INSERTED)
   BEGIN    
       PRINT 'DELETE'
       UPDATE b
       SET nrOfCopies= nrOfCopies-1
       from book b
       inner join DELETED d
         on d.isbn = b.isbn
   END
   ELSE IF NOT EXISTS(SELECT * FROM DELETED)
   BEGIN
       PRINT 'INSERT'
       UPDATE b
       SET nrOfCopies=nrOfCopies+1
       from book b
       inner join inserted i
         on i.isbn = b.isbn
   END
END