Robert Beekes (with the assistance of Lucien van Beek). Etymological Dictionary of Greek. Indo-European Etymological Dictionaries Online. Edited by Alexander Lubotsky. Brill. Brill Online.
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
Turner, Ralph L. (1973). A comparative dictionary of the Indo-aryan languages. 2. impr. London: Oxford U.P.
Robert Farren, PIE culture words collection, 2017
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This map shows the etymological links in a geographic context.