Heathenry can be very difficult to research due to the Germanic peoples of the past being largely illiterate and thus not leaving behind any records of their practices. This does not mean all is lost, however, though it is important to keep in mind the oldest records we have were written by Christians (and a Muslim). Here is a non-exhaustive list of some good sources on Germanic (mostly Norse) Polytheism! Feel free to add to this in the comments.

Books and Texts:
-The Poetic Edda (Norse)
-The Prose Edda (Norse, Christianised, Christian author)
-Nine Herbs Charm (Anglo-Saxon, Christianised)
-Old English Rune Poem (Anglo-Saxon, Christianised)
-Beowulf (Anglo-Saxon, Christianised)
-The Icelandic Sagas (Icelandic)
-Ibn Fadlan and the Land of Darkness (Muslim author)
-Heimskringla (Norse, Christianised)

Websites:
-The Longship (Anglo-Saxon)
-Wind in the Worldtree (Anglo-Saxon)

YouTube:
-Dr. Jackson Crawford (Not Pagan but scholar, Norse)
-Ocean Keltoi (Norse, should be noted that he was involved in controversey but is a wonderful academic source and he cites sources)
-Arith Härger
-The Heathen Household

There is a very good guide on Reddit explaining reconstructionism that I also recommend.