Carl Darling Buck 
Dictionary of Selected Synonyms in the Principal Indo-European Languages [PDF ebook] 

Originally published in 1949 and appearing now for the first time in a paperbound edition, Buck’s Dictionary remains an indispensable tool for diachronic analysis of the Indo-European languages. Arranged according to the meaning of words, the work contains more than 1, 000 groupings of synonyms from the principal Indo-European languages. Buck first tabulates the words describing a particular concept and then discusses their etymological and semantic history, tracing changes in meaning of the root words as well as presenting cases indicating which of the older forms have been replaced by expressions of colloquial or foreign origin.