Creating SQL relationships in SQL server 2008!

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SQL-Server-2005

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I have created several tables like stating at the below.

CREATE TABLE AUTHOR(AUTHOR_NUMBER VARCHAR(6),LAST_NAME VARCHAR(25),FIRST_NAME VARCHAR(25), PRIMARY KEY (AUTHOR_NUMBER))

CREATE TABLE BOOK(BOOK_CODE VARCHAR(6),TITLE VARCHAR(50),AUTHOR_NUMBER VARCHAR(6),PAPER_BACK BIT,BOOK_TYPE VARCHAR(25),UNITS_ON_HAND INT,PRICE MONEY,PUBLISHER_DATE DATETIME, PRIMARY KEY(BOOK_CODE))

CREATE TABLE BOOK_ORDER(BOOK_CODE VARCHAR(6),CUSTOMER_NUMBER VARCHAR(8),ORDER_DATE DATETIME, PRIMARY KEY (BOOK_CODE,CUSTOMER_NUMBER))

CREATE TABLE CUSTOMER(CUSTOMER_NUMBER VARCHAR(8),LAST_NAME VARCHAR(25),FIRST_NAME VARCHAR(25),CUSTOMER_ADDRESS VARCHAR(50),PHONE_NUMBER VARCHAR(10), PRIMARY KEY(CUSTOMER_NUMBER))

CREATE TABLE PUBLISHER(PUBLISHER_CODE VARCHAR(8),PUBLISHER_NAME VARCHAR(50),ORDER_DATE DATETIME,NOTE TEXT, PRIMARY KEY(PUBLISHER_CODE))

After that I wanted to create a relationship between Author table Author_Number & Book tables's Author Number and I have declared the following SQL statement in MS SQL server 2008 as the following.

ALTER TABLE BOOK ADD CONSTRAINT FK_AUTHOR_NUMBER FOREIGN KEY (AUTHOR_NUMBER) REFERENCES AUTHOR (AUTHOR_NUMBER) ON UPDATE CASCADE ON DELETE CASCADE)

But when I executed the statement I got the following error.

Msg 102, Level 15, State 1, Line 1
Incorrect syntax near ')'.

Please try to solve my problem. Thanx