This dictionary, with more than 1000 articles, provides a comprehensive survey of all important aspects of Celtic religion and culture, covering both the prehistoric continental Celts and the later, medieval culture that found written form long after the Celts had settled in the British Isles. This is a slightly revised and updated English translation of a book which was published in German in 1994. It is a pretty impressive achievement with entries ranging from two lines to a page, from very general introductory essays to specialised topics. There are linguistic derivations of names, lots of cross-references and bibliographies.
Dictionary of Celtic Religion and Culture