By calculations from earlier works in James Lee Burke’s popular series, fictional detective Dave Robicheaux should be past 80. His age is inconsistent in the sequence, but he’s a senior citizen in the latest, titled simply “Robicheaux.” He’s working as a sheriff’s deputy in New Iberia, La., and refers to himself in the narrative as “semiretired.” His colleagues call him “Pops” — among several other less complimentary epithets — but his energy and acuity are undiminished.