meta_query
needs to be an array of arrays – have a look at the code sample in the Codex again.
So, for your example:
$args = array(
'post_type' => 'pf_cookbook',
'meta_query' => array(
array(
'key' => 'pf_cookbook_recipes',
'value' => '5',
'compare' => 'NOT IN',
'type' => 'NUMERIC'
)
)
);
);