I've always loved Medieval literature, and though I don't read nearly enough fantasy as I should (I do like Tolkien, think I'll like David Eddings and even Robert Jordan, like Patrick Rothfuss thus far, really not a Terry Brooks fan though) I feel there's a connexion between those two that I can appreciate: warriors on horseback with swords fighting entire armies, adventuring, slaying dragons and rescuing queens. Love it! This book lets me feel that way without being pretentious or overdoing the whole “this world isn't like our own” bit that perturbs me; Medieval literature didn't care to make such disclaimers and was so bold as to suggest dragons could be killed in our world. And if not, who cares, it's fiction! At least Heinlein has the decency to say “we're leaving this world, from now on, anything goes.”
And of course, there's something special about the way Robert Heinlein writes. Mostly its his ideas for novels; they seem less like they come from the demand of a publisher or demand from the public and more from his own will to write about his passions in life. And even when he's firmly rooted in a fantasy story, you still get touches of his ideals for society, government, combat, relationships, sex and everything else he loves to talk about.
Robert Heinlein's first novel after he wrote Stranger in a Strange Land (a book that defied social norms and used his love of science fiction to relate a more passionate topic of his, that being sex) is the author's only work of fantasy.
But really, who expects to pick up a Heinlein book and read about dragons slain? Or magical powers used by an Empress to make a young warrior believe in himself and wield a sword with enough confidence that he can storm a highly fortified castle? It is still him, yes, but it is different. Not different in a way that makes you think he is trying too hard to re0define a genre; no it is actually quite basic fantasy stuff, but different because he has no other work like this.
If I were a harsh critic, I'd say it doesn't work, or that it doesn't fit. But I'm a Heinlein fan, and I say it's just good fun. About the only thing I didn't like is that need for an object, something physical that people use or need to possess or are trying to destroy. Plots that revolve around something so simplified as a physical, tangible object being obtained/destroyed really put me off, and it's really the only one of his works I've read to this point that uses that technique.
There is also a pretty lengthy bit at the end of their marriage and it reads like an instruction booklet on how to avoid a counselor with your spouse and still be happy. Maybe it drags on a bit too much and maybe I just get bored when he lacks violence or nudity for 10 pages.
Overall though it is definitely not what I expected when I picked it up, but what I like about the fantasy genre is how it reminds me of Medieval literature, and this has some pretty good parallels to that whilst still being absolutely Heinlein.