I found something that may be classified as plagiarism:
I have a Spanish book (translated) on "Nordic Mythology", by Heinrich Niedner. According to the back cover Heinrich Niedner (1875-1932) was a Danish philosopher and humanist, and the book was published in 1915. Well, I was unable to find any mention on the net of this Heinrich Niedner, other than this same book.
But, by translating some simple sentences to English and searching them, I found
this book, titled "Norse Mythology" and written by
Rasmus Björn Anderson (1846-1936), who was an American professor of Norwegian origin, and who wrote the book in 1875. From what I could see, the book I have is the exact translation of this English book. Where does this apparently fictional Heinrich Niedner come from? And what is the reason of this invention? The Spanish book was published in 1997, when the English book was not public domain yet... but if Heinrich Niedner were real, it wouldn't have been public domain either!