If this is the wrong sub for this question, please redirect me to the right place.

I was recently reading a book, and I was struck by the fact that some passages were literal translations of passages from another book. That other book is not cited in any way and the passages are presented as the author's work. The author is not a historian or anything like that, and the book is a combination of her diary and of the historical context around the events she witnessed. Only the latter is plagiarized. The book exists only in Russian, while the book I'm fairly sure she copied from has no Russian translation.

While I doubt anything can be done in this specific situation, what is usually done when plagiarism in an academic work is suspected? Especially when it's an issue of passing off a translation as one's own work.