Elvis Costello has always been an idiom savant, pin- balling through arsenic-laced pub rock (My Aim Is True), amphetamine-addled soul (Get Happy!!), and highbrow chamber pop (The Juliet Letters). His latest showcases another readymade style: dirt-floor Americana. Pairing with producer T-Bone Burnett (who helmed 1986’s rootsy antecedent King of America) and a distinguished pickup band of country heavyweights, he gives his typically fussed-over tunes a tent-revival authority. With alchemical highlights that include back-porch foot-stompers (“Hidden Shame”), torchy weepers (“I Felt the Chill”), and a tenderhearted, set-closing waltz (“Changing Partners”), Secret testifies to the merits of aging gracefully.