Well you can always go straight to the lore - for the Norse, The Eddas (poetic and prose) and the Sagas. Those are a great place to find Their stories. I agree with the above poster, make offerings and open yourself up. There's loads available online for free for Norse here: http://www.northvegr.org/
As for the Celts...well there is no one Celtic pantheon. I don't know a lot about the Celts but I do know there aren't many if any pan-Celtic gods. You'll probably need to narrow it down at first, to Irish or Welsh or Gaulish or Manx...For Welsh there's the Mabinogion, The Tain and the Book of Invasions for the Irish...and that's about all I know. You can check the CR FAQ: http://www.paganachd.com/faq/
for more info.