Very intriguing ideas here, and ones that, if valid, have major bearings on Beowulf and a whole host of interrelated topics (Iron Age Scandinavia, the "adventus Saxonum," the spread and preservation of oral legend). I'm both sympathetic to and excited by these ideas—but their substantiation requires a much greater treatment of the evidence and secondary scholarship than is provided by this slender book. Not all of Shippey's conclusions necessarily follow from the evidence, and there's some cherry-picking—none of which means Shippey is wrong, but, again, the claims need to be weighed and examined with a lot more depth.
Shippey is one of my favorite authors, and his works on Tolkien, medievalism, and medieval literature have been personally formative (and long a source of both pleasure and edification). Some hallmarks of his writings on full display here: his engaging and at times wryly witty style; his perspicacity; and his ability to show you what's exciting and intriguing about the subject matter.