Pasting the following HTML input into the “Body” field of a “Book page” set to split on <h1> and <h2>:

<p>This text is the introduction.</p>
<h1>Page 1</h1>
<p>Here is my text for page 1.</p>
<h2>Page 1a</h2>
<p>This is page 1a.</p>
<h2>Page 1b</h2>
<p>This is page 1b.</p>
<h1>Page 2</h1>
<p>This is page 2 and the last line of text.</p>

This will create the following five nodes as Drupal book pages linked together in a Drupal book structure:

Node 1 Title: Set explicit by user
Node 1 Body: <p>This text is the introduction.</p>
Node 1 Parent: top level
Node 1 Weight: 0

    Node 2 Title: Page 1
    Node 2 Body: <p>Here is my text for page 1.</p>
    Node 2 Parent: Node 1
    Node 2 Weight: -15

       Node 3 Title: Page 1a
       Node 3 Body: <p>This is page 1a.</p>
       Node 3 Parent: Node 2
       Node 3 Weight: -15

       Node 4 Title: Page 1b
       Node 4 Body: <p>This is page 1b.</p>
       Node 4 Parent: Node 2
       Node 4 Weight: -14

    Node 5 Title: Page 2
    Node 5 Body: <p>This is page 2 and the last line of text.</p>
    Node 5 Parent: Node 1
    Node 5 Weight: -14