Three Hearts And Three Lions (1961)
A clichéd but highly entertaining stab at fantasy.
Danish World War II partisan fighter Holger Carlsen is suddenly transported out of dire straits into the Middle World, an alternate universe combining bits of medieval history and loads of traditional European folk tale elements.
Carlsen, reinvented as a messianic knight, encounters all the usual suspects in a series of adventures as he crosses the alien landscape: a second-rate witch, a bunch of evil upper crust full-size elves living in Faerie, the obligatory unicorn, a riddle-loving giant, a town-terrorizing werewolf, a rather nasty mermaid, a nearly unkillable troll, even Morgan le Fay, the chief bad guy.
He picks up a few comrades along the way, all of whom you could do without: the occasionally annoying dwarf Hugi, who's around to provide some not very comic comic relief; the thinly portrayed teenager Alianora, who can turn herself into a swan; and later the slick-talking Carahue the Saracen.
Hugi and Alianora speak in unconvincing archaic English while the engineering-trained Holger speculates about scientific explanations for all the weirdness, which like everything renders the proceedings a bit silly.
There's also some moderately annoying Christian mumbo jumbo towards the end.
But Anderson is such a skilled writer that he pretty much pulls all of this off - it's a well-paced tale, imaginative but believable if you accept its hokey premises.
Heinlein should have taken some notes when he wrote the somewhat similar testosterone-fest Glory Road.