Unlike the reserve_confirmation hook that runs at the exact time the reserve confirmation email is sent, this hook fires after the reservation confirmation is sent. It is directly after reserve_confirmation.

 * @param $inventory_id
 * @param $data
 * @param $subject
 * @param $message
function my_custom_function($inventory_id, $data, $subject, $message) {
   // As an example how to get the item info, you could do the following...
   $item_info = new WPIMItem();  // Call new instance of WPIMItem class
   $item_info = $item_info->get( $inventory_id );  // Get the item information with inventory id
   $name = $item_info->inventory_name;  // Example of how you would get the name of an item

   // Do whatever you want from here!

add_action('wpim_reserve_confirmation_sent', 'my_custom_function', 10, 4);