Keep Circulating the Tapes: The BBC does not have the rights to release their recent radio dramatization of The Once and Future King as an audiobook, due to the rights of the novel being held by Disney (The Sword in the Stone) and the estates of Lerner and Loewe (Camelot). They gave the BBC the rights to broadcast their radio drama, but not to commercially release it.
Reality Subtext: White wrote the majority of the series, including The Book of Merlyn, during the darkest days of World War II. It definitely shows, if you're looking for it. The scene with the ants in The Book of Merlyn is an explicit parallel to totalitarian regimes and Nazism in particular. And Mordred's faction in The Candle in the Wind are described in ways intended to compare them to Nazis. Plus there's Merlin's angry lecture to Sir Kay in The Queen of Air and Darkness, although that's less subtext than outright text.
Screwed by the Lawyers: The rights to The Once and Future King are divided between Disney who did the animated film of The Sword in the Stone and the estates of Alex Loewe and Alan Jay Lerner who created the musical Camelot. When the BBC decided to dramatize the book for Radio 4, they had to get permission from both of them to do it. However the BBC does not have the rights to release their production as an audiobook.