Steinbeck's Arthurian tales was spurred on by his childhood in Salinas, California. As a child, he used to play as a knight and he made his little sister play his squire. He dedicated the book to her.
I am generally favorable to a lot of what Steinbeck did with his treatment, other than "vamping up" Morgan le Fey, which seemed a bit too dime store novel of a characterization to me.
Otherwise he does a great job getting into the psychology of the knight characters. The published version also includes a great number of his letters on his process, showing how Steinbeck himself was on a sort of spiritual grail quest in trying to get to an authentic version of the telling.
If you are into the Arthurian literature as it jumped from the medieval to the modern, this is a great book to add to your collection.
Last note: be forwarned that Steinbeck never really finished his book. He spent years doing the research, but never really wrapped the stories with a bow. Just appreciate the tales he does tell, and how he tells them. You already know how the story ends.