I’m afraid I’m not inclined to go through the process with every culture I can think of but here are some quick suggestions.
For Norse, you might try Dr. Stephen Mitchell as he teaches Scandinavian folklore courses at Harvard and Dr. Heather O’Donoghue as she teaches Old Norse and Icelandic Literature at Oxford university and is able to read the old languages.
They have a good few books between them to last you, and I assure you the next best way to find another book/article to read is to check their citations. They will be basically handing you a list of writing scholarly enough for them to reference.
However, for Celtic, I would suggest contacting tchy for his recommendations. I believe he has taken some courses at his university regarding the subject so might have a quite amazing list of books on hand.