I was surprised at how much I enjoyed The Fall of Arthur. I've never been a fan of Tolkien as poet and, as a rule, skim through the examples that crop up in his prose or that are reproduced in the History of Middle-earth volumes. But I was intrigued by the subject and by what Tolkien may have made of the Matter of Britain (Sir Gawain and the Green Knight doesn't count since it's a translation of an existing poem). 
 Unfortunately, The Fall of Arthur is incomplete. Tolkien only completed four cantos (in several versions which Christopher Tolkien exhaustively presents) but what's there suggests an original retelling of the war between Arthur and his son Mordred. The first cantos opens with Arthur fighting in the unmapped east when he learns of Mordred's treachery; the second introduces Arthur's son, his motivations for rebellion, and the queen and her motivations; the third cantos takes us to Lancelot, who languishes in Benwick mourning his fate; the fourth cantos describes Arthur's initial landing at Romney in Kent and the ensuing battle with the rebels (not Camlann, though; Tolkien never got that far). 
 I was especially interested in Tolkien's portrayal of Guinever. As I read the poem, she's a cold, grasping, shallow woman who even in the face of the ruin of the king's dreams shows no remorse: 
 / His heart returned 
 To its long thralldom / lust-tormented, 
 To Guinever the golden / with gleaming limbs, 
 As fair and fell / as fay-woman 
 In the world walking / for the woe of men 
 No tear shedding.... 
 In her blissful bower / on bed of silver 
 Softly slept she / on silken pillows 
 With long hair loosened, / lightly breathing, 
 In fragrant dreams / fearless wandering, 
 Of pity and repentance / no pain feeling, 
 In the courts of Camelot / queen and peerless, 
 Queen unguarded.... 
 (II.25-30, 32-38) 
 / But cold silver 
 Or glowing gold / greedy-hearted 
 In her fingers taken / fairer thought she, 
 More lovely deeming / what she alone treasured 
 Darkly hoarded. / Dear she loved him 
 With love unyielding, / lady ruthless 
 Fair as fay-woman / and fell-minded 
 In the world walking / for the woe of men.... 
 From war she shrank not, / might her will conquer, 
 Life both and love / with delight keeping 
 To wield as she wished / while the world lasted; 
 But little liked her / lonely exile, 
 Or for love to lose / her life's splendour. 
 In sorrow they parted. / With searing words 
 His wound she probed / his will searching. 
 Grief bewrayed her / and greed thwarted; 
 The shining sun / was sudden shaded 
 In storm of darkness.... / In pain they parted.... 
 (III.49-56, 97-106, 109) 
 I would recommend this for the Tolkien and/or Arthurian lit reader.