XmlNode.SelectSingleNode syntax to search within a node in C#

I want to limit my search for a child node to be within the current node I am on. For example, I have the following code:

XmlNodeList myNodes = xmlDoc.DocumentElement.SelectNodes("//Books");
    foreach (XmlNode myNode in myNodes)
    {
         string lastName = "";
         XmlNode lastnameNode = myNode.SelectSingleNode("//LastName");
         if (lastnameNode != null)
         {
              lastName = lastnameNode.InnerText;
         }
    }

I want the LastName element to be searched from within the current myNode inside of the foreach. What is happening is that the found LastName is always from the first node withing myNodes. I don't want to hardcode the exact path for LastName but instead allow it to be flexible as to where inside of myNode it will be found. I would have thought that using SelectSingleNode method on myNode would have limited the search to only be within the xml contents of myNode and not include the parent nodes.