Lancelyn was an appropriate middle name for a man who wrote about Lancelot. Truth be told, the only prose from Mr. Green that I've read is compiled in King Arthur and His Knights of the Round Table. It's been quite a while since I read the volume, so I'm going to make this brief.
Green is a very good writer from what I can remember. He was a peer of Tolkien, the Father of Epic Fantasy, which makes him pretty cool by default. His characters came to life in a way that rivals many written today. I recall a great amount of emotion in his work, albeit the memory is somewhat faint. The epilogue nearly brought me to tears (this I distinctly remember) through it's magically ominous delivery and tone.
I must delve into the writing of Roger Lancelyn Green more in the future. For now, I regard him as the best writer of King Arthur stories I've encountered, yet my exposure in that area is limited to the point where such a statement is somewhat wooden. I'd definitely recommend those stories to anyone interested in Arthurian lore or low fantasy. I expect Green's other works are solid as well, based upon his pedigree.