Most available books are monographs published between the 1920s and 1970s by Romanian authors later translated into English, which all echo a strong state-approved communist version of history and aren't all that useful as secondary literature. I know there are several anthologies dealing with (eastern) european countries and the collapse of their political systems (mostly), but unsure which have decent material on Romania in particular.
These should get you started:
Beer, K. P. (1983). Zur Entwicklung des Parteien- und Parlamentssystems in Rumänien, 1928-1933: Die Zeit der national-bäuerlichen Regierungen. Frankfurt a/M: Peter Lang.
Livezeanu, I. (1995). Cultural politics in Greater Romania: Regionalism, nation building & ethnic struggle, 1918-1930. Ithaca: Cornell University Press.
Clark, R. (2015). Holy Legionary Youth: Fascist Activism in Interwar Romania.
Maner, H.-C. (1997). Parlamentarismus in Rumänien (1930-1940): Demokratie im autoritären Umfeld. München: R. Oldenbourg.
Miege, W. (1972). Das Dritte Reich und die deutsche Volksgruppe in Rumänien 1933-38: Ein Beitrag zur nationalsozialistischen Volkstumpspolitik. Bern: Herbert Lang.
Nagy-Talavera, N. M. (2000). The Green Shirts and the others: A history of Fascism in Hungary and Romania. Iaşi: Center for Romanian Studies.
Watts, L. (1993). Romanian Cassandra: Ion Antonescu and the struggle for reform, 1916-1941. Boulder: East European Monographs.