Three Hearts and Three Lions by Poul Anderson
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A modern fantasy retelling of the story of Ogier the Dane one of the peers of Charlemagne, and a "king in the mountain". The book is interesting in certain respects: the author has clearly synthesized and interpolated a lot of medieval epic and other literary sources and the allusions are sometimes quite cleverly layered. The book is also of historical interest as it's the origin of a number of modern fantasy tropes, such as rubbery regenerating troll made famous by D&D.
There are some more-than-offsetting problems, though. The book is an early example of the "modern man of science mysteriously displaced through the planes into a fantasy world" trope, but that's both good and bad. Lazily deployed, that trope is, frankly, pretty tiresome. It has been used to good effect, for example, in some of Michael Moorcock's Eternal Champion books, but here the trope is very poorly explained with none of the psychedelic grandeur to be found in Moorcock's (admittedly later) novels.
There are also some plotting problems: the story moves along at a gripping pace, but then suddenly stops dead at what seems like the end of the third act of a five act play, and handwaves away the actual climax of the story. It also doesn't help that the book's portrayal of woman and general gender politics have aged like milk (nothing scaldingly horrifying, but a view of femininity that would have been right at home on 50's television).
So if you're really into Poul Anderson or Carolingian mythos, give this one a shot, otherwise, I'd advise a pass.