Julius Caesar's
De Bello Gallico is an obvious source. Google on "caesar druids" for more resources. The
Agricola and maybe the
Germania of Tacitus just might be useful. (I think that I have uploaded translations.)
And here are some links:
http://www.intute.ac.uk/artsandhumanities/celtic/
http://www.asnc.cam.ac.uk/resources/celtic.htm
http://www.digitalmedievalist.com/urls/
However, probably the most interesting current scholar on the Celts is Professor Miranda Green (aka Miranda Aldhouse and Miranda Aldhouse Green.) I would search Amazon for a list of her books. Her "The Celtic World" is
very good. And her other works are well worth reading.
Another prominent scholar in this field is Barry Cunliffe (an Amazon search will be fruitful).
One thing to be aware of is that there is a lot of revisionism going on in Celtic Studies at the moment.
In the early 20th century, archaeologists used to suppose that celtic artefacts found in Austria, France and Britain meant that there was a movement of peoples from the East to the West. It is, of course, feasible that goods were traded, so that widespread artefacts do not necessarily mean widespread migration of peoples.