Sir Gawain and the Green Knight is one of the first books written around the King Arthur
and the Knights of the Round Table
-type story (of which there are many; it was a very popular literary subject in the Rennaissance, as it was an era of change, and the King Arthur storyline was a good way to interweave paganistic traditions and new christian beliefs
.) At the base of the story is the ancient belief in a promise and the christian ethic of forgiveness.
Sir Gawain is sitting at his King's table, doing his thing, when a large green knight comes in and challenges anyone there to a duel one year from the date. Gawain, being the gallant hero he is, agrees to the duel for his King, and basically spends a year screwing around and worrying about this battle. When he gets to the home of the big tree-hued guy, he tries every way to squirm out of it. He ultimately gives in and lays his sword down, only to be forgiven by the generous green thing, teaching Gawain a lesson both of forgiveness, following through on your promise and honor.
I read Raffel's translation, but I believe J.R.R. Tolkien also did a translation.