On Belletristica, you have the option to add contributors to your books If somebody helped creating your book, you should always credit them properly.
You can very easily do this in your book's settings, by clicking on "Contributors". Here you'll see a variety of roles, from "co-author" over "cover design" and "editing" to "special thanks".
Just start typing the name of the person you want to mention, and select the appropriate card from the list of suggestions.