What is is this time? Schubert lieder? Balkan fusion? Elvis Costello's genre-hopping can be exhilarating and, often, exhausting. But here he's in one of his comfort zones, rootsy Americana, reunited with T Bone Burnett, who produced the similarly flavored 1986 ringer King of America. Recorded in Nashville in three days, it's tight and uncluttered, with fiddle and dobro accenting jaunty bluegrass-folk corkers such as "Hidden Shame."The music brings out the terser side of one of pop's most prolix lyricists, with some spectacular results: The closer, "Changing Partners,"is a waltz-time weeper so unfussy and timeless, you half-suspect Costello found it under a rock on an Appalachian hillside.