The Legend of Sigurd and Gudrún

Two long alliterative poems by Tolkien retelling the Norse Volsung legend (the story of Sigurd, Fafnir, and the Niblung treasure), edited by Christopher Tolkien and published posthumously in 2009.

It uses the same Norse source material as Wagner -- the Poetic Edda and Volsung Saga -- but Tolkien's version is a philological reconstruction rather than an operatic adaptation. Tolkien is known to have disliked Wagner's treatment of the mythology.

Not necessarily -- Christopher Tolkien's introduction and commentary provide the necessary context. However, familiarity with the Volsung Saga or Poetic Edda will enrich the reading considerably.

The Children of Hurin story in The Silmarillion is consciously modelled on the Sigurd/Kullervo tradition. Reading this volume shows the Norse source material that Tolkien was both translating and adapting when he created Turin Turambar.

Somewhat -- the story is self-contained and does not require knowledge of Middle-earth. However, it is written in alliterative verse and requires more patience than Tolkien's prose fiction. The extensive editorial commentary by Christopher Tolkien helps considerably.