The first version of Thomas Nugent’s translation in 1750 was based on the original 1748 edition,  supplemented by lists of corrections forwarded to him in 1749 while the translation was in progress. Later on, Nugent incorporated additions to subsequent French editions without specifying which ones, nor for that matter did subsequent publishers usually specify which of the previous editions of Nugent they were copying. To my knowledge, no one has ever sorted out exactly what variations exist in the innumerable reprints of Nugent editions. The translators of the Cambridge edition (1989), for want of a critical edition, used the 1758 edition (Amsterdam : Arkstée et Merkus, t. I–II) as presented in the edition of Œuvres complètes by André Masson (Paris : Nagel, 1950, vol. I).
The original 1748 edition of L’Esprit des lois is an unreliable text, not to mention that Montesquieu made some additions and corrections which were incorporated into the edition of 1750.  A letter by Montesquieu to Grosley on 8 April 1750 identifies Huart’s 3-volume edition as “l’édition la plus exacte”. The posthumous editions of 1757 (?) and 1758  for the first edition of the collected Œuvres de M. de Montesquieu included many more, but also many corrections made by the editors which cannot be authenticated. The major addenda of these editions are annexed to this text. This translation is based on a critical version of the 1750 text as established for the Société Montesquieu by Caroline Verdier, Pierre Rétat and Catherine Volpilhac-Auger.
The Spirit of Law is a work of history as well as theory, and the author observes scrupulous standards of documentation. Montesquieu read very broadly in ancient and medieval history. The Greek authors he read mostly in Latin translation, and his mastery of Latin gave him access to a millennium and a half of legal compendia, capitularies, edicts, canons and charters of many peoples. Many of these notes can be readily clarified with the on-line search engines presently available. An abbreviated table of Montesquieu’s commonest references is appended.
For translations of Montesquieu’s numerous quotations from Latin, I am most grateful for many suggestions from Catherine Volpilhac-Auger. Translations from Tacitus have often been based on that of Alfred John Church and William Jackson Brodribb, available on line. For the many passages of medieval law in the later books, I have frequently looked for help from the authors of the Cohler, Miller, and Stone translation.
The reference edition
I am using the somewhat amplified edition of 1750 published as Genève : Barrillot & Fils but actually by Paris : Huart et Moreau. This base text in three volumes can be found on the web via the following links :
Readers are invited to write me at email@example.com with corrections or further suggestions.
Académie = Dictionnaire de l’Académie françoise (Paris, 1762).
Furetière = Antoine Furetière, Dictionnaire universel (La Haye and Rotterdam : Arnout and Reinier Leers, 1690).
Catalogue = Catalogue de la bibliothèque de Montesquieu à La Brède, by Louis Desgraves et Catherine Volpilhac-Auger with the collaboration of Françoise Weil, Cahiers Montesquieu 4, 1999. Available on line as the Bibliothèque virtuelle.
OC = Montesquieu, Œuvres complètes (edition in progress, 1998– ), Oxford : Voltaire Foundation and Naples : Istituto italiano per gli studi francese ; (since 2010) Lyon : ENS Éditions and Paris : Classiques Garnier,
Trévoux = Dictionnaire universel françois et latin, known as Dictionnaire de Trévoux (Trévoux, 1743).