An incredibly neglected topic in English.
There's a few books on the politics, but I have found nothing at all on the campaign in which Romania crushed the new Soviet Hungary.
I have translated a chapter from Colonel Bujac's book (Paris, Charles Lavauzelle & Cie, 1933) dealing with the Romanian war with Soviet Hungary. It's not particularly long but is densely packed with information. I wouldn't take his word for the politics though – stick to his military stuff.
I have also translated some material from the French official history series relating to this war. There is quite a lot of material, but not particularly even in its coverage. Again, it is provided for military details, not the politics.