Parts of the poems are missing--lost to time--but large portions of them were preserved in a volume called The Prose Edda by a dude named Snorri Sturluson, in the year 1200-something. In that volume Sturlson apparently has prose versions of some of the poetry along with the poetry fragments, which helps to infill the missing bits. According to the considerable discussion about Tolkien's versions of these poems (Lays), which Christopher Tolkien takes from his father's notes, lectures, letters, and marginalia, Tolkien did not believe that some of the story elements were original to the poems, but later additions or edits, added by later poets or bards, to fill in missing information or to make them more appealing.
Tolkien has essentially pulled the stories apart and tried to reassemble them in their original form, and when that's not possible, he tries to re-create the missing bits in a form more faithful to the original poet's style and intent. He then translates them into modern English, using that same Old Norse meter and alliterative verse scheme.
But why? Why would Tolkien bother?
What becomes clear from the time and effort he put into the study of these lays, Tolkien believed that the literature of what he called the 'North' was just as compelling, dramatic, and important as the Greco-Roman Classics, among others, that we've all grown up with. His discussions on the value of Beowulf, for example, changed the way that both scholars and readers approached this poem: as a work of art by a talented poet, not just an interesting historical document.
His study of this story, and stories like it, also inspired his writing in the Silmarillion and The Lord of the Rings. And not just Tolkien, the story of lost Rhine gold is the inspiration for Wagner's Ring Cycle, and other stories as well. The story is of heroism, love and love lost, war, Valkyries, Norse gods, rivalry and tragedy.
And Sigurd and Gudrún... they may love each other, but one gets the distinct feeling that they aren't fated to be together.
Yeah...kinda like that.
This book isn't just a compelling translation of some old Norse epic poetry, its a retelling of one of the most famous stories from the north. Its also a peek into the mind of J.R.R. Tolkien, and what inspired him. A little dry in places, but I feel like I'm learning something worthwhile. I may even look up old Snorri.
Read this book. Yeah, I said it.