All the sites listed below are ones I have found useful and refer to in my posts.
The Rune-Poems: Icelandic, Norwegian, and Old English
Germanic Mythology (texts)
Norse Mythology for Smart People
Sacred Texts (Norse and Celtic)
Deo Mercurio is a site about the Roman god Mercury and his Romano-Celtic cult.
Noemi Beck’s thesis on Celtic goddesses is a good summary of scholarship on Romano-Celtic goddesses.
CELT: Corpus of Electronic Texts
Roman Inscriptions of Britain is a very useful website, good search engine.
The Corpus Inscriptionum Latinarum is a little less easy to use, but useful for continental Europe.
THEOI.com for Greek mythology
The Greek Mythology Link is focused on myths in art and literature, so it’s heavy on quotes from ancient Greek writers.
Ancient Egypt Online for the Gods and Goddesses
The Ancient Egypt site, the section on Pharaonic Religion
We Are Star Stuff 
Unfortunately your book Brigantia is nowhere available in Germany.
There is a PDF anywhere?
Michael
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It’s not in stores, but you can buy the online version at https://www.lulu.com/en/en/shop/sheena-mcgrath/brigantia-goddess-of-the-north/ebook/product-1ynmm5dk.html?page=1&pageSize=4
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