The difficulty is that the Norse wrote almost nothing about their religion while they were practising it. Runic inscriptions are short and formulaic. What survives is largely Icelandic and largely Christian.
The Prose Edda was written around 1220 by Snorri Sturluson, a Christian chieftain, lawspeaker and politician, roughly two centuries after Iceland converted. He wrote it as a handbook so that poets could still understand the mythological references traditional verse depended on. He is systematic, entertaining and the single richest source, and he is also arranging into a coherent narrative material that may never have been coherent. His prologue explains the gods as human kings later mistaken for divinities, a standard medieval device for handling pagan matter safely.
The Poetic Edda is a collection of anonymous mythological and heroic poems preserved in the Codex Regius, a manuscript of about 1270. Some of the verse is older than the manuscript; how much older is argued over. The sagas, written in the thirteenth and fourteenth centuries, describe pagan practice at several removes and with literary purposes of their own.
Outside observers add a little. Ibn Fadlan, an Arab envoy, watched a Rus ship burial on the Volga in 922 and recorded a human sacrifice. Adam of Bremen wrote about Uppsala without going there.
Archaeology supplies what the texts cannot: hammer amulets, the Oseberg and Gokstad ship burials, the Gotland picture stones, hall sites with deposits of animal bone, and place-names compounded with divine names. Where the texts and the ground disagree, current scholarship generally follows the ground.