The background is this: a Dane named Hnaef goes to visit his brother-in-law Hengest, a Frisian at a place called Finnsburg or something, in Frisia, they have to stay the winter, there's a quarrel, some deaths, an attempt at justice which breaks down so that everyonew ends up killing everyone. (Or at least that's what happened, but I may have scrambled up the names)

In Beowulf, after Grendel has been slain, a scald entetains the heroes of Hrothgar's court by singing a lay about Finn and Hengest. Trouble is, old northern poet were addicted to a practice called "kennings," refering to common persons, places and things with images and allusions drawn from myth and heroic legend. If you knew the whole set of stories, the code was self evident.
Trouble is we don't. Scholars have spent life times puzzling out the meaning of this and similar passages. In the case of Finn, all we have is the highly coded Passage in Beowulf and a 50 line fragment of an Anglo-Saxon poem a 19th century scholar found tucked into a book on something else entirely in some private library (this is just on of many examples of how precarious the survival of our stock of ancient manuscripts has been, I believe that both Beowulf and the Welsh Mabinogion were also accidental finds, of a few or even just one original manuscript.) Anyway, this 19th century dude made his own type script of the Fragment, then lost the original, a situation that scandalizes scholars. Is the typescript faithful, what clues from the original are no longer accessible, etc etc.
At Oxford, the Anglo Saxon philologist was expected to lecture on Finn from time to time, which Tolkien did app three times in his career, and maybe publish something on it, which Tolkien, it seems, never got around to (no surprise there!)
So this book is a posthumous stiching together of portions of three, widely separated specialist lectures, compiled by the guy who, unless I'm wrong, now does that sort of thing at Oxford.
The book has a neat cover, two Viking-type warriors, glowering while facing away from each other. But the inside is, well, inaccessible. It'll sit there on the SF/Fantasy shelves for a while, methinks.
One thing that would have helped would have been to include the Passage and the Fragment (always in caps, by the way). I do have a copy of Beowulf, some time I may dig it out and check out the Finn passage, probably not today...
One thing that caught my eye: controversy swirls around the word "eoton:" enemy? Is it to be understood as refering to generic or specific enemies, to unknown third parties, to the Jutes, or (my eyes light up) to giants, ettins, ents....Hmmm
: Note also that "13th Warrior" movie (espec in original book version, "Eaters of the Dead, by Crichton) retells the Beowulf legend in a semi-historical setting.