Good to see this guide back, will be an excellent reference point for many roleplayers.
Something being from the RPG does not make it inherently non-canon; the ‘RPG is not canon’ quote directly says that it’s not that black and white. WoW may contradict the RPG, or it may add onto it—hence, best refer to WoW and not the RPG books.
In conclusion to that & to what Agtherian said:
https://wow.gamepedia.com/Glyphic_Letter