Byzantium is a hell of a travelogue of medieval Europe. From the green hills of Ireland the reader is taken on a voyage through Scandinavia, Eastern Europe, the title city, and even the Middle East. The strength of Lawhead's Pendragon Cycle, for which he is best known, was its authenticity; here, that same strength is in evident supply. One caveat I will offer is that some readers may be put off by Lawhead's preachy Christianity; the ending left me with the uncomfortable feeling that I was reading a morality play.
The very best children's fantasies manage to successfully evoke a feeling of fantastical magic that is at odds with the scientific magic found in so many fantasies written for older readers. The Earthsea books have precisely this sort of magical magic; they also have the requisite archetypal characters and exotic locales expected of a children's fantasy. However, they also have surprisingly subtle themes and plots, and better writing than is usually seen in fantasy of any stripe. But then, you knew that; these are classics that have demonstrated appeal to, as the saying goes, "readers of all ages."