When Michael Wallis informed us that he was writing a book about Wild West outlaw Billy the Kid, we hoped that Wallis would separate the reality from the myth in Billy the Kid’s story.

It wouldn’t be an easy task. Since his death at age 21 to a bullet from Pat Garrett’s gun in 1881, Billy the Kid’s image has been distorted by yellow journalists, hack writers, half-baked Hollywood films and old-fashioned gossip. Some claim that Billy the Kid was a psychopath who killed dozens of men. Others — especially Hispanics — say he was a Robin Hood of the Southwest.

Many Billy the Kid storytellers seemed all too happy to follow the famed line from the film “The Man Who Shot Liberty Valance”: “This is the West, sir. When the legend becomes fact, print the legend.”

Fortunately, Wallis labored earnestly to sift fact from fiction in “Billy the Kid: The Endless Ride” (W.W. Norton, 328 pages, $25.95). Wallis drew on his many years in journalism to flesh out the Kid’s background. The book contains more than 50 pages of footnotes and references. Unless an unexpected cache of Billy the Kid documents is unearthed, this book will likely be the definitive work on the outlaw.

Wallis leavens the book’s scholarly tone with fascinating nuggets of information. (One example: An old windmill in the future Route 66 town of Las Vegas, N.M., was used for hangings so much that boys started hanging their dogs in imitation.) Wallis’ meaty prose — which helped make “Route 66: The Mother Road” a bestseller — also turns what could have been a clinical book into a more entertaining read.

The first 40 or so pages occasionally are slow reading, mostly because little is known of Henry McCarty (aka Kid Antrim, aka William Bonney, aka Billy the Kid) until he was nearly a teenager. Wallis and other experts strongly believe McCarty was born to an Irish woman in New York City. The family kept moving west — hastened by his mother’s bout with tuberculosis — until they ended up in Silver City, N.M.

Henry McCarty’s behavior during his early teens was little more than that of a mischievous kid’s and certainly not a killer’s. But his life went off the rails when his mother died of the TB and his stepfather, William Antrim, abandoned him. With little or no supervision, McCarty roamed the streets of Silver City and became a thief. McCarty’s future as an outlaw was sealed when the skinny kid, detained for larceny, escaped the local jail by shinnying up the chimney. That’s one accurate element of the future legend — Billy the Kid was an escape artist.

McCarty became a saddle tramp, roaming the territories of New Mexico, Arizona and the Texas Panhandle. He took legitimate ranch work when he could, but also stole livestock and gambled.

Billy the Kid is held in high esteem by Hispanics is because he spoke Spanish fluently and respected the culture. He was a good dancer and a ladies’ man with the senoritas. He was a symbol of resistance against white power brokers who derided the Spanish-speaking natives. “He was their El Chivato, their little Billy, a champion of the poor and oppressed,” Wallis writes. Billy the Kid’s deep connection with Hispanic culture has been long overlooked, and Wallis gives it its due.

McCarty changed his name to Billy Bonney and was caught up in the Lincoln County War. The story of a violent New Mexico turf battle between competing business interests and corrupt power brokers is too complicated to recount here. It led to a homicide rate in Lincoln County that was more than 40 times the national average. But of 50 people indicted in the conflict, Billy the Kid was the only one convicted.

Billy the Kid has only four confirmed killings to his name. Two were arguably in self-defense, and two occurred during his famed escape from the Lincoln County Jail shortly before his death. It’s hardly the stuff of a hardened murderer.

But Wallis is reluctant to call a criminal like Billy the Kid a scapegoat. “The young man may have been used and abused by the many duplicitous people that he encountered in the final years of his life, but he himself also established a critical role in establishing his own identity,” Wallis wrote.

Still, when Sheriff Garrett’s bullet finds its mark at the end of the book, it feels like a tragedy. Billy the Kid had been kicked around by bad luck and bullies all his life. He did his best with pluck and optimism, but the forces against him were too big to overcome.