Norse Mythology all comes from the Eddas and Sagas. Since the Norse and Vikings, were descendants of Teutonic Peoples, I've also included some references to Germanic mythology, as well as Anglo-Saxon as it's all related, afterall all 3 groups worshipped Odin for instance.
Source Lore: The Eddas (poetic & Prose) and Sagas (Heimskringla, Gesta Danorum, the Ango-Saxon Chronicle, etc.), Skaldic Poetry, Tacitus' Germania, Beowulf and other Anglo Saxon sources like the Lacunga. Also the eye-witness reports from travelers such as Ibn Fadlan.
Scholarly works about the ancient culture: Grimm's Teutonic Mythology (all 4 volumes), Fletcher's Barbarian Invasions, Dubois' Nordic Religion in the Viking Age, Simek's Dictionary of Northern Mythology, texts from Hilda R Ellis-Davidson tend to be the staples of many a library.