I am developing a new project that includes many post types each with their own custom fields, I would like to know if it is possible to organize in the admin menu so as to have a mother post type and a sub menu with child post types.
If you'd like to move the "Books" post type under the menu item for the "Book Shops", you can go to the "Books" post type's settings screen, and add "edit.php?post_type=book-shop" in the input field below the "show_in_menu" option.
( as shown in the attached screenshot )