XML STRING Passed To Stored Proc

I am using SQL SERVER 2005 and ASP.

Would anyone be kind enough to give me a solution to the problem below please?

If for example in the stored procedure looks like this:

-- Initialize the SQL Server XML stored procedures
EXEC sp_xml_preparedocument @xmlDoc OUTPUT, @xmlString

Insert into @TableBook
SELECT * FROM OPENXML (@xmlDoc, 'books/book') WITH
(
bookNameVARCHAR(100) 'name',
authorVARCHAR(100) 'author'
)

The question is supposing there is a node called 'Surname' but I am not adding 'Surname' to 'Author'. If I want to check 'Surname' value DOES exists and I want to replace it with 'Author' or if 'Surname' does not exist, just use 'Author' node instead.

I am not sure how you can use IF statement to check a node and put the value into SET @surname and then put something like author VARCHAR(100) '@surname' OR author VARCHAR(100) '@Author'. Just like switching it around.