For example...

<text>
<front> <!-- titlepage, etc here --> </front>
<body>
 <div type='book' n='I' xml:id='JA0100'>
 <head>Book I.</head>
    <div type='chapter' n='1' xml:id='JA0101'>
    <head>Of writing lives in general...</head>
        <!-- remainder of chapter 1 here -->
    </div>
    <div n='2' xml:id='JA0102'>
        <!-- chapter 2 here -->
    </div>
 <!-- remainder of book 1 here -->
</div>
<div type='book' n='II' xml:id='JA0200'>
 <!-- book 2 here -->
</div>
<!-- remaining books here -->
</body></text>