In a Nashville bookstore, to the tune of steam hissing from a latte machine and laptop taps of nearby browsers, she speaks in a low voice, yet communicates urgently. Her voice never rises. Her music never rattles rafters or crashes like cymbals toward the high notes in a power chorus. Her tempos shuffle and trudge more than they dash. And her songs? They’re about as idiosyncratic as anything in the wide world of “popular music.”