What is the importance of Shei'rah in the novel, The Unicorn Sonata?

Shei'rah is an interesting place and a singular achievement in fiction: Beagle defines it in terms of music, in sounds described in words. Everywhere in Shei'rah, music, forests, meadows, and streams all have music, all singing to Joey. Shei'rah is in a dimension apart from that of earth, but it is forever attached to earth; it drifts so that its border with earth occasionally shifts but never parts from earth, perhaps because the Eldest, the unicorns, are linked to humans.