In World War I, trained ‘mercy dogs’ roamed among the wounded on the battlefields of France. A soldier could call one over and hold it for comfort while he died. It’s one of the affecting factoids that stray through Rin Tin Tin, New Yorker journalist Susan Orlean’s exhaustive study of the most famous German shepherd in history.

Rin Tin Tin was a puppy when an American soldier called Lee Duncan rescued him from a shelled French farmhouse. Duncan, a dog-trainer, took Rinty home to the United States, where the dog’s spectacular physical prowess and pensive expression (“as if he were viewing with charity and resignation the whole enterprise of living”) won him a Hollywood contract. He became a superstar of the silent screen: no longer a dog but a legend, “amplified and projected on the boundless scope of a public dream”.

When Rinty died in 1932, the talkies had arrived and mute heroes of all species had fallen from grace. But Duncan went on breeding and training German shepherds. Each successive generation included a Rin Tin Tin, not necessarily descended from the original animal – like all mythological figures, Rinty was now immortal. He returned to his fans with television, where he was one of Orlean’s earliest memories: “A big dog bounding across the screen to save the day.”

But the Rinty who mesmerised Orlean was only 8 inches tall. A forbidden object, it stood on her grandfather’s desk when she was a child. Orlean believes that the cheap figurine represented something precious to the old man: the America he had glimpsed, like a promise, in “some dark little theatre in eastern Hungary when he first saw Rinty race across the screen”.

Such moments of personal connection between Orlean and her subject are wonderful but rare. Orlean’s research is impressive, but it produces a book that is lifeless for long stretches. The myriad plotlines – the feuds, the action doubles, the inevitable lawsuits, all detailed down the years – constitute a formidable deterrent to reading pleasure. Ideas glitter and beckon: our complicated, shifting relations with animals, for one. But they are seldom pursued, let alone convincingly.

When Orlean finally got to play with the figurine on her grandfather’s desk, she snapped off one of its legs. In the postmodern present, the fetishised object returns, on – where else? – eBay. Orlean buys it happily, but is later disappointed: a plastic dog is no substitute for the “bittersweet weight” of memory. These were warnings to heed. Rin Tin Tin retrieves everything that can be known about its subject, but facts can’t stand in for mythologies. The true object of Orlean’s nostalgic quest is the lost country of childhood, where it’s always summer and “Yo, Rinty!” calls up the dogs we loved and outlived.