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De Vaan's Etymological Dictionary of Latin is perhaps the most up to date one. He also bases his articles on the other main etymological dictionaries, like Walde-Hofmann and Ernout-Meillet. Sometimes I would have liked his articles to be a bit more expansive, but that would have required a lot more time to write. It is available on paper and online.


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If you allow the answer to be a bit wider in scope than the question (this is not Physics.SE, and we have this leeway in humanities), and talk about how linguistic evidence in general, not only written or traditionally transmitted evidence, which is the scope of your question, I should mention a book that is considered somewhat controversial among ...


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Michael Weiss's Outline of the Historical and Comparative Grammar of Latin is the most comprehensive, well-researched, and balanced historical grammar of Latin. There is a website with addenda and corrigenda maintained by the author. It is available on paper only; the second, corrected printing was released in 2011.


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