The author apparently wanted to make an overview of literal evidence for pre Christian practices all over Europe. His area goes from the far North to Greece and from Ireland to the Easternmost parts of Europe.
The subjects are thematic. Landscape, elements of that landscape, statues, shrines and temples, rituals, calendar, Gods, priests and important points in life and in the year.
The book reads a bit like the mythology books of 150 years ago. As in: ‘The Romans did this and the Slavs this.’ Dowden mostly uses written sources and looks at them critically. For Germanic information he mostly uses Jan de Vries.
So “European Paganism” became a bit of an inventory. You can check what sources are available on a wide variety of subjects and in many cases Dowden sketches how credible the source is. There is not much new information, but some of his sources are not the best known.
Dowden does refer to Dumézil and his theory several times and here and there has an uncommon opinion such as stating that Thor in many cases is the God of the Thing (p. 286).
Even though the author seems positive critical towards paganism and shows the colored information from Christian sources, he does say on page 2017: “If, on the other hand, we are convinced, as I am, that the pagans were wholly deluded in supposing various gods to exists and that ontologically, in the cruel light of day, they were worshipping nothing.”
2008 Routledge, isbn 0415474639