Get Value from XML File based on two child node values
===

I have an xml document and need XPath expression to select node values based on two child node values.
XML File structure is:

    <catalog>
    <book id="123"> 
    <title>Book123</title>
    <author>Author123</author>
    <genre>Genre123</genre>
    </book>
    .
    .
    .
    </catalog>
I want to select book based on title and author. I've used this expression but it shows an exception which said this expression has some invalid token. I didn't figure it out how to solve this.
Expression is :

    "book[title='" + txtBookTitle.Text + "' AND author='" + txtBookAuthor.Text + "']"

Any body help me in this regard ?