the book dirctory instances is like this:
<book_directory>
  <first_directory>
     <first_directory_name>computer book  </first_directory_name>
     <second_directory>
        <second_directory_name> databases </second_directory_name>
          <third_directory_leafnode> oracle </third_directory_leafnode>
          <third_directory_leafnode>  sql server 2000 </third_directory_leafnode>
          <third_directory_leafnode>  db2  </third_directory_leafnode>
     </second_directory>
     <second_directory>
        <second_directory_name> programe language </second_directory_name>
          <third_directory_leafnode> java </third_directory_leafnode>
          <third_directory_leafnode>  c++ </third_directory_leafnode>
     </second_directory>
     <second_directory_leafnode> hardware book  </second_directory_leafnode>
  </first_directory>
  <first_directory>
  ...
  </first_directory>
</book_directory>

book detail information instances is like this:

<orcale>
  <book>
    <name> a </name>
    <author>...</author>
    ...
  </book>
  <book>
    <name> b</name>
    <author>...</author>
    ...
  </book>
  <book>
    <name> c </name>
    <author>...</author>
    ...
  </book>
  ...
</orcale>
<db2>
  <book>
    <name> e </name>
    <author>...</author>
    ...
  </book>
  <book>
    <name> f </name>
    <author>...</author>
    ...
  </book>
   ...
</db2>
<java>
  <book>
    <name> h </name>
    <author>...</author>
    ...
  </book>
  <book>
    <name> g </name>
    <author>...</author>
    ...
  </book>
   ...
</java>