In response to anything stating that Shepherd Derrial Book is superfluous, useless, and not contributing anything to the Serenity crew, I vehemently disagree. Even without the complete revelation of his backstory or the resolution of his character arc, he still contributes something. And I’m about to inarticulately express what that is and in the process forget a bunch of important points.
As Nathan points out in both Browncoats Unite and in the Companion book, the crew can be interpreted as people representing what Mal has lost in himself during the War. Book, obviously, represents spirituality. Not religion, per se, or any particular institution, but the concept of personal growth, reformation, changing of oneself for the better, that such a thing is possible, that one is not trapped by what they have done and what they have experienced. He also represents belief, not necessarily the religious type, and the power believing and what it can accomplish. Mal, after the War and as we see him now, may believe in things (like freedom) but not with the same fervor, with the same mentality that one can win a war against a larger power simply because one believes deeply in the cause one thinks just. Book is there to remind Mal of that.
Book, often acts as a diffuser in crew conflict. Though Mal is captain and the highest authority, Book is one of the crew members who is a steadying force. His age, perceived wisdom, and position as a Shepherd makes him often a diffuser in conflict. Book is also a moral compass; he soothes those who may doubt themselves, guides all the younger characters into considering the right thing to do or what is the most right thing (see the next point), and though he often annoys them by mincing an argument he calls to light that though he himself deals in rights and wrongs nothing is black and white (even things as foreboding as the Alliance, and this whole show runs on gray morality). To say it again, Book most often calls attention to the fact that in many places there are no absolutes, and things are much more nuanced than they often appear to other characters, and that much of everything is shades of gray.
Throughout the series, it is hinted that Book may not have such a glittering past. It seems like his entire career as a Shepherd is precipitated by regret at his former life. (It’s most vividly expressed in the comic Downtime and actually revealed in The Shepherd’s Tale, but I don’t really agree with Tale, so that’s that.) His attempts to move past his former life and change himself echoes something in two other major characters: River and Mal. All three attempt to move past what he has done, what was done to her, what he has lived through, and their arcs, in however amount they were completed, are about breaking free of it and not being trapped by it. They simply find different ways that work for them, and they reflect out to one another that it is possible.
To reduce usefulness on the crew down to action heroics or job description is a disservice to the dynamics of the crew. And, really, the crew is not held together by a single person. They’re held together because they’re a group of individuals who find in the other eight a kind of wisdom, solace, comfort, companionship. The crew is a complicated balancing act of ideologies, beliefs, priorities, value systems, worldviews. Just as his own flaws and shortcomings and limitations are balanced out by the others, Book, for his part, brings foresight, nuance against absolutist thinking, caution, morality, conscious attempt to reform, second chances.
Yes, he may not do a lot in the way of narrative action or helpfulness on a heist or have an actual job description, but that’s not what actually what being on the crew means in the end.
PS, the exchange between River and Book in “Jaynestown” regarding the Bible doesn’t ‘bum her out’ or ‘piss her off’ in any way. River was effectively destroying his Bible, and he patiently explains to her that she needn’t treat it as a historically or scientifically accurate document. He tells her that it’s okay the Bible is wrong about things like physics because that’s not the point. He wasn’t, in any way, a dick about it; he doesn’t berate her or anything. And, hey, maybe the Bible has sentimental value to him.