Those plans for peaceful seaside holidays? They never really work out well, at least not in the world of classic British mysteries.   Retired professor Andrew Basnett, for example, envisioning little more than sandcastles and the blessings of a pale English sun on his pale English skin, is startled to meet his nephew, Peter, on the beach. He’s yet more startled (and not entirely thrilled) when Peter gets him invited to dinner with a celebrity novelist. And he’s extremely startled when the novelist’s sister-in-law is shot in the summer house and nephew Peter seems the likeliest man for the job. We have often suggested that Andrew Basnett should be known as “Mr. Marple,” because the series’ village settings and pinpoint plotting—and its canny, creaky sleuth—are so pleasingly reminiscent of the tales of St. Mary Mead. But they feature, in addition, a gentle wit that (dare we say it?) Ms. Christie could only dream about, and this final installment is a perfect exemplar.