There is an excellent book on Celtic culture which explores the Celts in Europe (Hungary, Austria, etc) called The Celtic World, editied by Miranda Green. The edition I have, published by Routledge and reprinted 1996, includes information on the Celts in Eastern Europe (chapter 30, by Elizabeth Jerem). This is a scholarly work with lots of pictures of artifacts etc. The book is 800 plus pages and covers origins, art, religion and a host of other topics, with essays by many different scholars.