Alexander is best known for his juvenile fantasy series The Chronicles of Prydain; he may have written other books as well, but that's the series I'm aware of. You may know a bit of the story from the forgettable Disney flick The Black Cauldron; if you saw it and disliked, don't let it prejudice you. The books are rather better, and, like all quality children's literature, entertaining for adults as well. Light entertainment, sure, but not to be sneezed at.
The Chronicles of Prydain is the story of the coming of age of a young man named Taran in a land based roughly on Welsh and Celtic mythology. He starts out as Assistant Pig-Keeper, and ends as...but that would be telling. He makes a number of friends along the way, including Hen Wen the oracular pig, the furry and lovable Gurgeh (who was terribly misserved by the Disney movie); and he faces the and defeats Arawn the dark lord.
I first encountered Prydain when I was a teenager; I remember a girl of whom I was fond arguing that she liked Prydain much better than The Lord of the Rings, and that anyway they were pretty much the same story (after all, "Arawn" is pronounced almost the same as "Sauron"). I didn't agree with her then, and I don't now, but the two series do share something that goes deeper than the surface. Like Frodo and Sam, Taran wins through not by being special or super-powerful or because of magic goodies but through plain old everyday courage, faithfulness, and sweat. (Or perhaps he is special; those qualities are not as common as they should be.) Alexander's tales, like 's, have a strong moral point of view, a sense of proper, honorable behavior, that's absent from so much literature these days--and they manage it without being either "PC" or preachy.
The books are certainly suitable for teenagers; I would even try them on grade schoolers, though they might be too advanced. But what do I know; I first read The Lord of the Rings when I was nine.
If you like this...
...you might also like The Dark is Rising Sequence, ' Chronicles of Narnia, and 's Earthsea Trilogy. Oh, and that fellow 's stuff, too.'s
The Chronicles of Prydain