Classic overview in print
Walker A history of the law of nations v. 1. From the earliest times to the peace of Wesphalia, 1648 (1899) Internat 510 W185c
MPEPIL divides its into overview of the History of International Law into the following:
Machiavelli on International Relations by Thirty Years' War, 1618-1648
The above is a subject search term in SOLO in its own right.
The Ashgate Research Companion to the Thirty Years War by 
Diplomatic Theory from Machiavelli to Kissinger by 
Literature and the Law of Nations, 1580-1680 by Francisco de Vitoria c1483-1546
The Spanish Origin of International Law: Francisco de Vitoria and his law of nations by Balthazar Ayala, 1548-1584.
Jean Bodin 1530-1596
Origins of Legislative Sovereignty and the Legislative State by Gentili 1552-1608
His key publications have been translated into English:
De legationibus libri tres Internat 750 G338a (Classics of international law, no. 12) considering role of ambassadors.
De iure belli libri tres (1598) (Internat 750 G338b (Classics of international law, no. 16) 3 because he considers when war can be justified ad bellum, rules during battle in bello and the roles of law after post bellum.
De armis Romanis has been translated more recently
The Wars of the Romans : a critical edition and translation of De armis Romanis by 
Roman Foundations of the Law of Nations by Grotius 1583-1645
The Rights of War and Peace by 
The Cambridge Companion to Hugo Grotius by