Adding new node to existing XmlDocument object

I have an xml of following format.

<BOOKS>
    <BOOK>
        <TITLE>book 1</TITLE>
        <AUTHOR>author 1</AUTHOR>       
        <PRICE>10.90</PRICE>
        <YEAR>1985</YEAR>
    </BOOK>
    <BOOK>
        <TITLE>book 2</TITLE>
        <AUTHOR>author 2</AUTHOR>       
        <PRICE>20.90</PRICE>
        <YEAR>1995</YEAR>
    </BOOK>
</BOOKS>

I have an Add(XmlDocument xDoc, Book newBook) method to add new book to the XmlDocument object that is passed to the Add(..) method. How can I do this.