As and addendum to my post on Tolkien's translation of Beowulf, I'll add a note about his Sellic Spell. A "sellic spell" as defined by Tolkien is a "wonder tale" or "fairy tale" and appears to have constituted a genre of Anglo-Saxon oral tales of which we have no attested examples. The word "syllic/sellic spell" comes from a description of the various entertainments in Hrothgar's hall in Beowulf. Tolkien theorized that a lost "sellic spell" might account for some of the "fairy-tale" elements in Beowulf (ie his great strength, comparisons with a bear, his odd follower Handshoe and opponents sea monsters, Unpeace and the ogre). Tolkien worked out this theory in the way you'd expect: he wrote his own version of the lost "sellic spell" about Bee-Wulf the bearish hero, his three friends, and the Ogre -in Old English. Both the Old English version of this fairy story and its Modern English translation are included in Beowulf: A Translation and Commentary. My wife has yet to set to work on the Old English version, but we read the Modern English version together and found it highly enjoyable.
Aside from its enjoyment value, Sellic Spell is also a valuable insight into Tolkien's scholarly mind. Much of the source material of Tolkien's area of study, much more so than in the cases of Greece and Rome, was lost to the ravages of time. As he says of the origins of the dragon's horde in Beowulf, it is beyond the reach of song -but not of imagination. Taking this as his rallying cry, Tolkien was willing to use imagination where source material failed. Indeed, much of his legendarium was an attempt to recover the lost mythologies and cultures of the ancient Northern peoples through imaginative reconstruction. If that idea interests you, I'd recommend taking a look at Tom Shippey's excellent study The Road to Middle Earth. If you want to see a great example of Tolkien welding together scholarship and imagination, I'd recommend that you pick up Beowulf: A Translation and Commentary and read Sellic Spell.