Rating this book is complex. On one hand, the subject matter is quite interesting, and showcases a great deal of thought and research that has gone into the book. There are some strong arguments made here, and beyond that, this introduced me to a lot of myth I personally was not particularly aware of, and so that has to count for something, no? The formatting is something I enjoy, although often the book seems to lack a great deal of detail.
At the same time, the book has key and distinct issues. To expand on the lack of detail, the text feels extremely rushed. The book, including its conclusion, is 183 pages long, and beyond this, includes three short appendices (two of which are of texts, the third of which is the index). Often it feels that Walter barely delves into any of the things the text brings up, which seems contradictory to the point of expanding the collective knowledge on Celtic myth in contrast to the general understanding of Greco-Roman contributions to Christian faith and practice.
Many of the the arguments here are therefore extremely unrefined, and this makes the book seem in places extremely weak. I don't know if this is entirely Walter's fault however, as I do not have the original French text, and it is clear that some translation choices harmed the book. For example, following a long discussion on why many etymological arguments about Carnival are false, the author then shifts to discussing the book's theory, that Carnival is related to the Italian goddess Carna, which seems just as specious as the rest. Only on page 79, does the translator feel the need to inform us, three chapters later, that in French, Carnival is Carnaval, with the word Carna explicitly in it. Not all of the arguments are so clearly absolved by deep translator error, however.
Additionally, the text seems to lament the loss of the diversity and depth to Celtic myth, but seems to have a simultaneous identity crisis. The text is not singularly about Celtic myth; the title is "Pagan Origins", not "Celtic Origins", and Walter constantly brings up a nebulous Indo-European progenitor faith to explain all mythological similarities between any pagan faith the book discusses, utterly ignoring their own uniqueness in favor of casting them all as unimportant elements to showcase the Monomyth. Not once does Walter ever consider convergent evolution at all; take bridges for instance, they cannot possibly just be seen as risky by many civilizations who came up with this threat assessment on their own, they must somehow all originate from some ur-bridge myth. Added to this is the fact that Walter has a very condescending tone when relating some beliefs and practices; for instance, it is the "primitive" belief of some Medieval scholars to not wish to write down names of divinities, which is a value judgement which does not properly fit into a scholarly analysis of religious practice.
Finally, Walter seems to struggle with beliefs that are not the explicit study of the book. Saint Christopher, who has a dog head, cannot possibly in any way be Anubis, because he is a ferryman psychopomp, and while true this may not 100% track with Anubis, Walter sees it fit to entirely ignore the fact that Anubis was imminently involved with the afterlife, what with being the Egyptian god of the dead and of funeral rituals. He also deeply confuses Judaism, and in turn Christianity; for example, he claims that Pesach (or Passover), is celebrated "in commemoration of a passage- that of the Hebrews over the Red Sea when they were pursued by the Egyptians in the Exodus". While this is indeed true, this is massively reductive of Passover and its meanings, and as early Christianity is an outgrowth of Judaism, feels like Walter does not have as good a grasp on the origins of Christian faith as would be necessary to dissect Christian practice.