Contribution by different author in author (non-edited) book
Couldn't locate previous posts about this issue.
It's possible to introduce contributions by various authors under "Book (edited)" type entries, but not under simple "Book" entries. But often there are annex texts, such as prefaces, translator introductions etc., which exist in books that we'll be normally referring to as one-author books and which don't necessarily have editor names in their credentials.
Is there any way to create a child reference under a book which is not edited? How should I solve this problem if I want to refer separately both to the book as author-book and to the introduction by different author?