This book consists of two short novels printed in the format of the old Ace Doubles, back-to-back with separate covers and upside-down in relation to one another. The Sword of Rhiannon was first published as half of one of those Ace volumes in 1953, bound with no less of a big name than Conan the Conqueror by Robert E. Howard. It was never one of my favorites of her work, perhaps because I was initially struck by Conan's overwhelming presence. It's set on Mars, but not the Mars of her main Eric John Stark sequence. There's a time-travel element and some of the descriptions are nicely done, but I didn't find it up to her usual standard. Divide and Rule was first published in John W. Campbell's Unknown (which was a fantasy companion to his Astounding SF) as a serial in the April and May 1939 issues. I don't know why it was in a fantasy magazine; it seems to be pretty straight forward sf to me. Perhaps ASF was full... It's an amusing story of a future world that's been re-set to a feudalistic state by alien conquerors, rabbit-like creatures who are hyper-intelligent by dint of special mind-enhancing helmets. So if you can get them to take off their hats... It's a fun but simple story, with a kind of abrupt ending. I was amused by the fact that my 1990 edition has a cover by A.C. Farley with an Interstate Highway sign in the background, but the story is from 1939... And the dedication reads: "To Bob Heinlein"! Fun stuff, but not the best from either writer.