In her memoir 'The Glass Castle,' Jeannette Walls wrote about being a Barnard College student in a class that covered the root causes of homelessness. When Walls tried to voice an opinion, her professor asked her, 'What do you know about the lives of the underprivileged?' Being young, cautious and fearful of exposure, Walls chose to shut up. That was then. In 2005, well established as a New York journalist, she published her account of the background she had worked so hard to hide. It began with the image of Walls, then living on Park Avenue, looking out the taxi window to see a homeless, rag-covered woman rummaging through a Dumpster and realizing that the woman was her mother. When a book is full of true stories like that one, it's hard to tell the dancer from the dance. What accounts for the vast popularity of 'The Glass Castle'? How much of its success is a measure of Walls' painful past and how much is attributable to her writing talent? Those questions are underscored by the arrival of 'Half Broke Horses,' Walls' second book about her family. Its subject is her maternal grandmother, Lily, who was known as Grandma Smith in 'The Glass Castle' and described in that book as 'a flapper.' And although Walls set out to tell the true story of her grandmother's life, she found herself filling in too many blanks for this to be nonfiction. So 'Half Broke Horses' is narrated in the first person and billed as a 'true-life novel.' 'Half Broke Horses' is hardly the story of a flapper. Lily Casey (Grandma Smith's maiden name) didn't exactly flap. She ranched. She taught. She played poker, broke mustangs and won races. She looked like Amelia Earhart at the time she took flying lessons, and 'Half Broke Horses' has the photos to prove it. Little Lily, born in a dirt dugout in 1901, can also be seen on the book's cover as the oldest of three raggedly beautiful kids, living in the wild West but looking like the essence of Walker Evans' Appalachia. Walls chooses to tell Lily's story in a punchy style, broken up into short chapters and punctuated by abundant cracker-barrel wisdom. Few episodes end without a zinger, aphorism or fortune-cookie one-liner. And those truisms are so relentless that even Rose Mary, who was Lily's daughter and eventually became Walls' trouble-plagued mother, gets sick of them. Rose Mary speaks for the reader when she finally complains about the 'Let this be a lesson' and 'Let that be a lesson' stories that she gets from Lily. But Walls has packaged Lily as a gumption-packin' ranch gal whose pluck never quits. It runs in the family: According to 'Half Broke Horses,' Lily grew up hearing the same lingo from her parents. 'Mom liked to say Buster could charm the sage off the brush,' Lily says of her brother. Two pages later, Lily's father says, 'First thing a horse needs to learn is to be a horse.' Dad quickly follows up with 'Most important thing in life is learning how to fall' and 'Hope for the best and plan for the worst.' The pert style of 'Half Broke Horses' is much more repetitive and grating than the more spontaneous-sounding voice Walls used to describe her own life. But the author comes from a family that knew how to lure horses using grain, not rope. And she has inherited a version of that skill. So she has managed to make her second book almost as inviting as her first, even though its upright heroine is never as startling as Walls' parents were. Though 'Half Broke Horses' fills Lily's story with dramatic events, from the suicide of her sister to her betrayal by a 'crumb bum' husband, this is a tamer, tidier book. And it has a less complicated soul. Lily lived much of her life in lonely, picturesque Southwestern settings, though she lived in Chicago long enough to marry a traveling salesman and find out that the marriage was a fake. (Crumb bum husband: 'Calm down or I'll drop you with a roundhouse punch, and you know I can.' Lily, whaling him with a purse that held a six-shooter: 'You go ahead, buster, you hit me and I'll charge you with assault as well as robbery and bigamy.') She took and lost numerous teaching jobs. She met Mr. Right. ('This here is Jim Smith. Some folks call him Big Jim.') And she traipsed out in public wearing a dress made of velvet curtains, just like Scarlett O'Hara in 'Gone With the Wind.' Lily's story becomes trickier after Rose Mary (whose name is spelled Rosemary in 'Half Broke Horses') is born. So does Walls' narrative challenge. She needs to make her two books dovetail, and to do it without too heavy a hand. Saying that Rosemary was a strong, wild thing, 'a little like a half-broke horse,' is on the blunt side. But Lily also gives more detailed evidence of Rosemary's big, rebellious personality, and she speaks frankly, thanks to the benefit of her granddaughter's hindsight. 'I began to worry that Rosemary was unfocused and that if we didn't stamp it out early, it could become a permanent part of her character,' Lily observes. 'Half Broke Horses' foreshadows Lily's fate by mentioning that she had a Geiger counter and liked to collect and save rocks that got it ticking. It also follows Rosemary into her marriage to Rex Walls, a dashing pilot, who calls Big Jim 'Pardner' (Big Jim calls Rex 'Flyboy'). When the Smiths coax Rex onto a horse and the horse throws him, Rex says prophetically: 'I found the gas. Now all's I need to do is find the brake.' Walls's readers already know that he will never find it. They also know that when Rex and Rosemary become the parents of a 'Little Critter' named Jeannette at the end of 'Half Broke Horses,' a pretty doggone good storyteller is born. PRODUCTION NOTES: 'HALF BROKE HORSES' By Jeannette Walls 272 pages. Scribner. $26. 