Tolkien is most famous for the Lord of the Rings and, my particular favourite, The Hobbit. I think every other book he's written pales in to insignificance when you think of him as a writer. He was, in fact, first and foremost, an academic. The Legend of Sigurd and Gudrún denotes something that Tolkien was obsessed and ultimately influenced by, and that is Norse mythology. Norwegian and Icelandic poetry was his forté and here Tolkien has devised his own version of ancient poems regarding the legend of Sigurd and the fall of the Niflungs.
Written in narrative poetry form with short stanzas, The Legend of Sigurd and Gudrún is quite difficult to get in to if your heart is not in it. If you love Tolkien and enjoy his often epic narrative style, then I'd recommend this book purely because his son (Christopher) has used Tolkien's own notes to add accompanying notes that explain what is happened during each section, which is invaluable. At first read the text is difficult to digest because of the archaic English that is used, but after you have grasped what is happening you can retrace your steps and revel in the story.
Like most things, I'd prefer this to have been written in novel format and I'm sure someone somewhere has done such a thing.