Robert B. Parker is nothing if not reliable. Back Story, the Boston scribbler's 30th Spenser novel in 30 years, puts the Shakespeare-quoting PI on the cold case of a woman killed during a 1974 bank robbery. The comfortingly formulaic plot leads Spenser to the doorsteps of unrepentant hippies and mobsters. As always, the character sketches are Ginsu-sharp, and the prose leaner than a George Foreman Grill'd steak (''So far it was a good day. No one had attempted to murder me''). Bonus for Parker devotees: a crossover cameo by Paradise, Mass., police chief Jesse Stone, antihero of another of the prolific author's addictive series.