SQL Instruction for 3 tables

Hi! I'm new in the forum.
I have a doubt in a SQL instruction.

I have 3 tables. Books, Authors and Books_Authors.
This is the content of the tables:

Books:
1 - Book 1
2 - Book 2
3 - Book 3
4 - Book 4

Authors
1 - Author 1
2 - Author 2
3 - Author 3
4 - Author 4

Books_Authors
1 - 1
1 - 2
1 - 3
2 - 2
2 - 3

What I want, is a query that, when I choose Book number 1, and want to add more authors, just return the authors 3 and 4 because 1 and 2 are already been used.
I tried to do it with subquerys but don't know how!

Thanks in advance!

SELECT dbo.xAuthor.AuthID
FROM dbo.xAuthor LEFT OUTER JOIN
dbo.xBooks_Authors ON dbo.xAuthor.AuthID = dbo.xBooks_Authors.AuthId AND dbo.xBooks_Authors.BookId = <book ID>
WHERE (dbo.xBooks_Authors.AuthId IS NULL)

this will return the unused author ids for the specified book ID

Thanks subeeshkn! It works!

Liliana