This book is not a retelling of the stories from Norse mythology, nor a general overview of the beliefs of the pre-Christian Norse (and other Germanic) people. Rather, it is an analysis of how the myths themselves may have evolved over time, and the place of the myths in the religious beliefs of people during the Viking Age (say around the year 900) and the conversion era (roughly around the year 1000 in Scandinavia).
The discussion of the sources goes into the problem of circularity: there are pictorial artefacts (e.g., picture stones) that appear to show scenes from well-known Norse myths. The pictures were made well before the myths as we know them were recorded, and thus provide evidence that the myths go back a long way before the myths were written down. But without the literary evidence -- especially Snorri's Prose Edda -- we would not know what the pictures were about. We use Snorri to interpret the pictures, then point to the pictures as evidence that Snorri's tales are authentic. Abram points out that this is not an invalid use of the evidence, but has limitations that should be kept in mind.
Abram questions some common assumptions about evidence. One idea I particularly liked is that grave goods do not necessarily tell us anything about beliefs in an afterlife. As a non-religious person myself, I can see the emotional appeal of burying a dead loved one with items identified with that person, although I do not believe in a hereafter. Nor do I think the reason a modern Christian would bury his mother with her wedding ring is because he thinks she will need it in heaven. Yet somehow we leap to the conclusion that early people provided their dead with grave goods because they expected them to use them in some further existence.
A good deal of the book deals with how myths developed over time, not just "organically," but in the hands of the skilled poets of the Viking and conversion periods (as well as in retellings after the conversion -- e.g., by Snorri and Saxo). Poems in the pre-Christian age may have been written, using traditional mythical material, but with new emphasis, new details, and in new combinations, to reflect the social and political realities of the time of writing. New stories may have been told about the old gods, within the framework of the existing mythology.
Not only did the mythology thus change over time, but Abram also points out that an authentic, traditional mythology is not the same as the religion of a group of people. Mythology and religion go together, but that does not mean that practitioners of a religion necessarily accept the mythology as literal truth. To again think of this in modern terms, many modern Christians do not literally believe that Jehovah created the world in seven days, some 6000 years ago; but the Christian creation story is still meaningful in their belief system and an important part of the culture of Christendom.
While this book is a serious scholarly study, it is accessible to the lay reader -- interesting and enjoyable to read as well as thought-provoking.