Child pages in books module
The book module has this bit of code in it to enable a user to choose which parent a node has:

$form = array(
  '#type' =&gt; 'select',
  '#title' =&gt; t('Parent item'),
  '#default_value' =&gt; $book_link['plid'],
  '#description' =&gt; t('The parent page in the book. The maximum depth for a book and all child pages is !maxdepth. Some pages in the selected book may not be available as parents if selecting them would exceed this limit.', array('!maxdepth' =&gt; MENU_MAX_DEPTH)),
  '#options' =&gt; book_toc($book_link['bid'], $book_link['parent_depth_limit'], array($book_link['mlid'])),
  '#attributes' =&gt; array('class' =&gt; array('book-title-select')),
  '#prefix' =&gt; '&lt;div id="edit-book-plid-wrapper"&gt;',
  '#suffix' =&gt; '&lt;/div&gt;',
);


Take a look at the description in particular. When you sub in the relevant variables, it reads:


  The maximum depth for a book and all child pages is 9. Some pages in the selected book may not be available as parents if selecting them would exceed this limit.


Emphasis mine.

So the limit intentional, it (necessarily) matches up with the maximum depth for a menu in the same system.

It would technically be possible to patch core and increase MENU_MAX_DEPTH to something larger, but bear in mind that other parts of the system rely on it (for example the number of p* columns in the menu links table), so it probably won't be as simple as all that.