XSD Uniq constraint issue

I have an issue with enforcing the uniq constraint on "BookName" in the following XSD. Can someone please let me know what is the issue and why it is not working.

<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" 
           targetNamespace="http://ift.tt/1DKET5S"           
           xmlns="http://ift.tt/1DKET5S" 
           xmlns:mstns="http://ift.tt/1DKET5S"
           xmlns:xs="http://ift.tt/tphNwY">
  <xs:element name="xmlExtraction">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="BooksNode" minOccurs="1">
          <xs:complexType>
            <xs:sequence>
              <xs:element type="xs:string" minOccurs="1" name="BookStoreName"/>
              <xs:element name="bookNode" maxOccurs="unbounded" minOccurs="1">
                <xs:complexType>
                  <xs:sequence>
                    <xs:element minOccurs="1" type="xs:string" name="BookName"/>
                    <xs:element minOccurs="1" type="xs:byte" name="versionNumber"/>
                    <xs:element minOccurs="1" type="xs:string" name="AuthorName"/>
                  </xs:sequence>
                </xs:complexType>
                <xs:unique name="uniqueBookName">
                  <xs:selector xpath="mstns:bookNode"/>
                  <xs:field xpath="BookName"/>
                </xs:unique>
              </xs:element>
            </xs:sequence>
          </xs:complexType>          
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

I am unable to enforce the uniq constraint on the bookname can someone please paste the full xsd which enforces the constraint.

Thanks a lot for your help