In case this hasn't been mentioned, the letters of Queen Marie of Romania to her mother between the years 1901 and 1920 were published a few years ago. These are the details of the publication:
"Queen Marie of Romania. Letters to Her Mother", edited by Sorin Cristescu, 2 vols., Bucharest, Tritonic, 2016. [Vol. I: 1901-1906, Vol. II: 1907-1920]
Sorin Cristescu has also published other editions of correspondence of the Romanian royal family.