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.
Alphapuggle PieFed
That Heathen Guy