Custom Field Query – Meta Value is Array

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'
                        )
                    )
); 

);