Reddit reviews Agricola and Germania (Penguin Classics)

Here is a good beginner's book on OHG.

Here is an OHG-English dictionary in PDF form.

Germania is perhaps the greatest source we have for insight into the Germanic tribal religion.

Jacob Grimm's Teutonic Mythology, while controversial, provides a lot of insight into lesser-known Germanic deities.

Even post-conversion works like the Kinder- und Hausmärchen can help us understand the remnants of heathen belief in German culture.