Starting with the Beowulf poem, the author connects the lady serving mead there with Germanic queens from the migration period and the prophetess Veleda who worked with Juluius Civilis in AD 69 to carve out a Gallic-Germanic empire. He shows (convincingly) how a Celtic institution was used by Germanic warlords to instil loyalty in the comitatus (warband) outside of tribal structures and he follows these structures up to the Viking era.
Some parts of the book are heavily academic, while others are easier to read for laymen such as me. I found his assertion that Odin is traced book to Hannibal and Sertorius (both one-eyed) intriguing and also how he connects Odin with Mercury and the veleda with the Gallic goddess Rosmerta.
The author leans both on old texts (especially Tacitus) and archeology to prove his points.
One piece of proof which was a gold nugget to me, was an ostracon (potsherd) found on the Elephantine Island on the Upper Nile with the name Baloubourg (Waluburg), who was a prophetess for the Semnones, which suggests that she could have been there with auxiliary troops from the same tribe.
Two other prophetesses, Aurinia and Ganna, are also mentioned in the text and with Wealhtheow and Veleda, they are also connected to the role of the volve (Norse seeress). And druids and their poet counterparts are also up for discussion in this book.