//check if the previous sibling node is an element node
function get_previoussibling(n)
{
x=n.previousSibling;
while (x.nodeType!=1)
{
x=x.previousSibling;
}
return x;
}

xmlDoc=loadXMLDoc("books.xml");

x=xmlDoc.getElementsByTagName("author")[0];
document.write(x.nodeName);
document.write(" = ");
document.write(x.childNodes[0].nodeValue);

y=get_previoussibling(x);

document.write("
Previous sibling: ");
document.write(y.nodeName);
document.write(" = ");
document.write(y.childNodes[0].nodeValue);

author = Giada De Laurentiis
Previous sibling: title = Everyday Italian