Normalization of library book table with multiple authors

I have a database structure with two tables:

• The first one is called Author, has two columns (AuthID, AuthName) and its Primary Key is (PK_AuthID).

• The second one is called Book and has three columns (BookID, BookTitle, AuthID). Its Primary Key is (PK_BookID), and has (FK_AuthID) defined as a Foreign Key that referes to Author table.

The question is: If one Book has more than one Author, what is the best design pattern to follow in order to maintain the second Author, and at the same time keep the Book table without NULL values?