Leonard van der Kuijp, “On the Composition and Printings of the Deb gter sngon po by ’Gos lo tsā ba gzhon nu dpal (1392–1481).” JIATS 2, 2006, pp. 1–46.
See ibid. (pp. 4–5): “The word deb gter/ther/ter, meaning “book,” is ultimately of Persian or Greek origin, and entered the Tibetan lexicon via Mongolian, when most of the Tibetan cultural area was under Mongol rule (1240 to the 1350s). The orthographic instability indicated by the use of gter, ther, and ter is by no means uncommon for loanwords we encounter in Tibetan writing. Why some Tibetans should have chosen to use this word for book rather than a bona fide word for the same from their own lexicon is a question whose answer is still outstanding. Probably, the reasons are not altogether different from the “oeuvre” / “writings” alternation the reader finds in this essay. It is a question of style and perhaps also an attempt to capture a different kind of elegance. The patina of the foreign is sometimes more attractive than the local.”
 B. Laufer, “Loanwords in Tibetan,” Toung Pao 17 (1916), 481–82, no. 140.