Blackwell's A COMPANION TO THE LATIN LANGUAGE, edited by James Clackson, consists of 31 chapters by a number of authors that examine Latin from both a synchronic and diachronic perspective. It covers the entire history of the Latin, from Proto-Indo-European to Neo-Latin. There's so much specialist focus here that we even get individual chapters on Epistolary Latin, Christian Latin, Legal Latin and the language of Latin historiography.
The strong point of this compendium is that it represents recent studies of Latin in the light of modern sociolinguistic theory. Many authors seek to find the diversity between the artificial unity of Classical Latin norms. J.N. Adams's work on geographical variation in Latin is taken into account, as are studies of social registers in the tradition of William Labov. Anyone interested in Latin would profit from this book. While it seems meant for those of a linguistics bent, I think this book would be important to all classicists in showing them that behind the classical canon was a real spoken language like any language today.
My only major quibble is a general disregard for Balkan Romance in Roger Wright's contribution on the Romance languages as a source for Latin. Even when that significant branch of Romance has something to contribute, he overlooks it, and Romanian gets only two brief and inconsequential mentions.