I'm sorry to say I'm a touch disappointed. :/ 
 I'd been looking forward to finally getting the backstory on Shepherd Book--as only hints of it were ever mentioned in the original series. Book is such a sagely paternal character, and his presence rounded out the crew of Serenity with a quietly powerful temperance. Yet, it was clear he wasn't always the man he'd become. 
 This graphic novel telling has a harsher cast to the art style than others in the series. Deep colors and hard shadows add grit but--for this reviewer at least--detracted from both the beauty and the ability to clearly discern between characters in a few scenes. The story itself is told in a series of flashbacks within flachbacks, creating a jerky narrative effect that felt disorienting to this reader on several occasions. 
 The content itself was intriguing. The idea of Book starting out as a runaway from an abusive drunken father, surviving for a time as a volatile petty criminal, had a strongly viable resonance. That he'd volunteer himself as (view spoiler)[ a double-agent for the Independence movement back before the war with the alliance even began was a fascinating twist. In essence, he was an uncaught mole trapped behind enemy lines; forced to stay there once his cause was crushed. Lost and alone, lacking a true identity, he becomes a drifter and a drunk. Until some combination of head-trauma, the kindness of strangers, and divine revelation awakens him to a new path and purpose. (hide spoiler)] Really it isn't so much the content itself as the execution that fell a bit short. 
 Side Note: The telling also seemed to reinforce what was repeatedly indicated in the TV series--that while Shepherd Book seems blatantly intended as a theologically-educated, Bible-toting representation for Christianity, his professed version of faith is ultimately works-based rather than grace-based. It's hard to say if that denotes a lack of understanding on Joss Whedon's part, or an intentional modification. Either way, one doesn't get the sense it's a malicious misconstruing. But those who actually subscribe to the Christian faith could understandably find this aspect frustrating.