From what part of the world would a person refer to love letters as letters of love?
I looked at all the blurbs for letters of love 1800–1811 book links at ngrams (for love letters,letters of love) and all of them are English (not American, not Australian, etc). At that time, it was a phrase used comparatively frequently by Richard Steele and Joseph Addison in their Spectator or Tatler; and used as frequently in like publications of the time.
More recently (ie, 1972–2008) letters of love is used mostly in titles of books, and those books appear to be from a variety of places: USA, Israel, Germany, Russia etc. But note that incidence of letters of love now is minuscule compared to that of love letters, whereas in the early 1800's it was about half.