Mitra-Varuna is the best place to go, in English, for Dumézil’s views on the first function of sovereignty, and its twofold nature: the first judicial and worldly, the second divine and supernatural. In French, Les Dieux souverains des Indo-Européens (Gallimard 1977) updates and expands many of these analyses, but that’s not a text available in English. The second volume of Mythe et épopée, translated as three English books – The Stakes of the Warrior, The Plight of a Sorcerer and The Destiny of a King – discusses related questions. Except for The Destiny of a King those English volumes are out of print, but the forthcoming revised edition of Mitra-Varuna is an attempt to bring some of this work back into circulation.
For the second, martial function of Dumézil’s tripartite analysis – sovereigns, warriors, producers – an English reader could look at The Stakes of the Warrior, but I think a better place to start is The Destiny of the Warrior. Again, the book is unfortunately out of print.
The book has an interesting publishing history – a 1956 text, a revised version in 1969 and a new edition in 1985. Each of these adds material to the version before, with some cuts. On this page I’ve provided an initial indication of how and where these changes were made. The English translation is of the 1969 edition, and there is about a third of the final 1985 text which is not available in translation.