For the products which are only "paperback", just create a custom field on the table product.
For the products with variants "paperback" and "hardback", add the variants.
To display the custom fields only on the "paperback" product, create a category, display the custom field only for this category, and add this category to your "paperback" products.
By this way the custom fields will only be displayed on the "paperback" products.
To have the same display than when it's variants, you just have to customize your product page by editing some css properties in HikaShop > Configuration > Display "frontend css file".