You said you're using v1.5.1, which is very old. I don't have a copy of that installed anywhere to test.
So all I can offer is a hypothesis based on using code that appears to work on v1.5.7, but trying to indicate how to hack it into the older structure of v151.
I figure there are two spots that need code changes. Note that these proposed changes will apply to ALL categories, not just to "some". You can put in your own restrictions by changing the code yourself.
includes/index_filters/overrides/default_filter.php (or maybe your "book" site is using a different filter already. In that case apply your changes there, adapting accordingly).
You'll need to insert come code above line 76 where $listing_sql begins,
Code:
$child_categories = array();
zen_get_subcategories($child_categories, $current_category_id);
and then change the highlighted line 84 to:
Code:
AND p2c.categories_id IN (" . implode(',', $child_categories) . ") " .
/includes/modules/pages/index/header_php.php
After the following section
add:
Code:
$category_depth = 'products';
Again: just guessing. This approach is kinda brash, and wide-sweeping. Your mileage may vary.
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