Java – Running XPath on child node

I'm trying to do a xpath lookup on nodes returned by xpath lookup, but it doesn't seem to work as I expected.XPaths executed on the child nodes of a document seem to be executd against hthe root node of the document (in the example, the inventory tag.), instead of the root of the provided node.

Am I missing something here? I'm new to XPath.

Also, please don't answer "just do //book[author='Neal Stephenson'/title". I have a legitimate use case, and this is a simplified example.

Code snippet

DocumentBuilderFactory domFactory = DocumentBuilderFactory.newInstance();
domFactory.setNamespaceAware(true);
DocumentBuilder builder = domFactory.newDocumentBuilder();
Document doc = builder.parse("src/main/java/books.xml");

XPathFactory factory = XPathFactory.newInstance();
XPath xpath = factory.newXPath();

Node book = (Node) xpath.evaluate("//book[author='Neal Stephenson']", doc, XPathConstants.NODE);
Node title = (Node) xpath.evaluate("/title", book, XPathConstants.NODE); // I get null here.
Node inventory = (Node) xpath.evaluate("/inventory", book, XPathConstants.NODE); // this returns a node.

book.xml

<inventory>
<book year="2000">
    <title>Snow Crash</title>
    <author>Neal Stephenson</author>
    <publisher>Spectra</publisher>
    <isbn>0553380958</isbn>
    <price>14.95</price>
</book>

<book year="2005">
    <title>Burning Tower</title>
    <author>Larry Niven</author>
    <author>Jerry Pournelle</author>
    <publisher>Pocket</publisher>
    <isbn>0743416910</isbn>
    <price>5.99</price>
</book>

<book year="1995">
    <title>Zodiac</title>
    <author>Neal Stephenson</author>
    <publisher>Spectra</publisher>
    <isbn>0553573862</isbn>
    <price>7.50</price>
</book>

<!-- more books... -->

</inventory>