Buck, C., D. (1949). A dictionary of selected synonyms in the principal Indo–European languages: a contribution to the history of ideas. Chicago: University of Chicago Press.
Matasović, Ranko. Etymological Dictionary of Proto-Celtic. Indo-European Etymological Dictionaries Online. Edited by Alexander Lubotsky. Brill. Brill Online: http://iedo.brillonline.nl.ludwig.lub.lu.se/dictionaries/content/proto-celtic/index.html
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This map shows the etymological links in a geographic context.