Modify a function without editing template

Oviously, you’ll have to edit something if you want to modify it. So, your best course of action is to create a child theme, with its own function file.

A filter, as you tried, won’t work, because there is no filter hook in the function that you are trying to modify. What you can do, is remove the action that is printing the footer and then add a new action on the same hook. Like this:

add_action ('wp_head','wpse252108_remove_add_action');

function wpse252108_remove_add_action() {
  remove_action ('book_landing_page_footer', 'book_landing_page_footer_credit', 40);
  add_action ('book_landing_page_footer', 'new_credit', 40)
  }

Because it’s unclear where in the flow template-hooks.php, the place where the original action is added, is loaded, it is possible that wp_head is too early to do the add/remove action. Perhaps wp_footer is better, though that in turn might be too late.