Shepherd Book remained shrouded in mystery throughout Firefly and the cinematic sequel, Serenity - but here's a breakdown of what the subsequent comics revealed of his backstory. Created by Joss Whedon, the science-fiction western ran for one season back in 2002. Despite its early cancelation by Fox, Firefly became a cult favorite and has remained passionately beloved to this day. Set in 2517, the series followed the eclectic, renegade crew of the titular Firefly-class spaceship and their equal parts heroic and criminal adventures. Serenity was released in 2005 and served to tie up some of the show's lingering threads.
Played by the late Ron Glass, Derrial Book first appeared in the pilot episode when he sought passage on the ship as part of his travels. His devoutly religious nature initially put him at odds with the ship's now-faithless captain, Malcolm Reynolds (Nathan Fillion). Despite that, he remained a regular presence during the crew's adventures - often serving as a spiritual and moral guide. Along the way, however, it was gradually revealed that Book had an antithetically intimate knowledge of criminal organizations and military strategy. He was also an extremely capable fighter and an expert marksman. The mystery surrounding Shepherd Book deepened even further when it was revealed that he had a high level of clearance within The Alliance. In Firefly's makeshift series finale, Book is also recognized as a threat and more than he seemed by Jubal Early - with the bounty hunter declaring, "That ain't a Shepherd".
Serenity served to leave that particular mystery unanswered. Even the character's dying breath was used to deny Mal (and the audience) the truth of his past. As a result, theories abounded for many years. The most prevalent of them was that he'd been an Operative of The Alliance, similar to the one played by Chiwetel Ejiofor in the 2005 movie. The truth, however, was definitively revealed in the 2010 comic book, Serenity: A Shepherd's Tale. The story revealed that Book had actually been born Henry Evans. Raised in an abusive home, music served as his childhood solace. After running away, however, he fell into a life of crime - making a name for himself as a master thief. Evans remained a career criminal until an armed stand-off with a police officer led directly to him being recruited by The Independence movement. Over the next four years, he would hone his already considerable fighting skills even further.
Soon after, Evans was chosen to be a long-term mole within The Alliance prior to the Unification War. It was then that he murdered the real Derrial Book with a garrotte and took the identity on as his own. His left eye was also surgically removed and replaced with a biorobotic camera and transmitter. It's unclear whether such a backstory was always planned for Book, but the detail ties with one of Firefly's most memorable quotes - when Book ominously described himself as "more or less intact".
Signing up for The Alliance, he rose rapidly through the ranks and eventually became an officer. This was accomplished via his willingness to use brutal methods when interrogating captured enemies. That willingness impressed his superiors and was also teased in Firefly, courtesy of a vision of Book's past experienced by River Tam (Summer Glau). Ultimately, however, he steered a massive number of Alliance forces into ambushes. His actions caused over 4000 fatalities and led to the capture of 300 soldiers. He also brought about the destruction of some of The Alliance's biggest spaceships, including the I.A.V. Alexander. It was dubbed the "single greatest disaster in Alliance history" and led to Book being discharged without trial.
He spent the next few years living on the streets. While eating a bowl of chicken soup at a shelter, however, he had an epiphany regarding his life and place in the world. The moment led him directly to the Southdown Abbey, where he officially became a Shepherd. After a decade, he left and became a traveling missionary. From there, Book went on to experience the adventures fans witnessed in Firefly and eventually led to his death in Serenity.