The character, almost always referred to as "Shepherd Book," is a Christian of an unknown denomination. The original script for pilot episode "Serenity" includes this scene establishment:
"We see, passing through frame, Shepherd BOOK. […] His clothes are plain and instantly identify him as some kind of Protestant minister."
Throughout the series, he makes references to Christian theology and consults the Bible. Shepherd Book, in the pilot episode for the series, indicates he has been living in the Southdown Abbey and has never been married. Throughout the series there are references to monks at the Southdown Abbey as well as Biblical quotes. He becomes the soul of the crew, as Kaylee is its heart. Shepherd Book makes references, in both the series and the movie, to having some level of insider knowledge about how the Alliance works. When Mal asks him about this he states: "I wasn't born a Shepherd," but declines to explain further. At one point while critically injured his identification card earned him immediate treatment from Alliance medical personnel when their CO had originally left him to die. When River was able to see into the hearts of the crew, it is hinted that he experienced a cruel and sadistic past. His biography is never fully explained.
In the comic Serenity: Those Left Behind, Book expresses concern that he is being corrupted by living on Serenity and leaves the ship. By the time the movie Serenity begins, he is living on the planet Haven. In the film, Book is killed by an Alliance soldier sent to destroy Haven after Serenity manages to shake an Alliance assassin known only as "The Operative." However, he was not a passive figure in the events leading up to his death; he defended Haven and "killed the ship that killed them." His last words to Mal was that he didn't care what Mal believed in, just that he believed in something. Spurred by the shock of his death, Mal dresses his ship in Reaver camouflage and sets off for Miranda, a planet behind Reaver territory.
His background is explored further in the comic book Serenity: The Shepherd's Tale. Book was born Henry Evans, a boy who was raised by an abusive father. He ran away from home and began life as a petty criminal before being recruited by the Independence movement and moving out to the Border Worlds. Long before the Alliance begins the Unification War, forward-thinking Browncoats assign Evans to be a long-term mole. He sheds his old identity by killing a random passerby and stealing his identification card, becoming Derrial Book. His Browncoat superiors keep tabs on him by surgically removing one of his eyes and replacing it with an artificial camera. He joins the Alliance military and quickly moves up in rank. Becoming an officer, Book intentionally leads a risky operation that results in a humiliating defeat for the Alliance. He is forced to retire from the Alliance military. It is implied that the Alliance covers up the loss, therefore not officially punishing Book. Homeless, he visits a soup kitchen and finds God while contemplating a bowl of soup. He takes refuge at an abbey where he becomes a shepherd before leaving on the Serenity to become a missionary.