This is a good read: http://www.amazon.co.uk/gp/product/1841194689/

Has lots of references to Roman and pre-Roman texts, and suggests most of them relate back to a single source. I don't have the book with me to check at the moment, but it's the best book on 'The Druids' that I've come across so far - very readable, and stacks of bibliographic references. The same guy has also written a companion volume on the History of the Celts which is good too.