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Concordats in the sense of a "treaty" were prominent in Pope Pius XI's papacy (cf. the Lateran Treaty) and in Pope Pius XII's also (Pope Pius XII was Pope Pius XI's secretary of state). See this list of concordats.
Also, there are some good books on the Catholic Church and State doctrine:
Catholic Church and Christian State: A Series of Essays on the Relation of the Church to the Civil Power vol. 1 & vol. 2 by a cardinal-archivist of the Vatican Archives, Cdl. Joseph HergenrΓΆther
Catholic Principles of Politics by Fr. John A Ryan