I know I am a little behind the rest of the world in having only just seen this film, but it does raise some interesting issues, some of which are relevant to contemporary events.
There are spoilers here, so please be aware that you might enjoy the film better before you read this. I want to explore some of the theological issues that the film raises, and there are a number, because it is a deeply theological film.
Firstly, the book is the bible. It is believed to be the last copy of the bible in a post-apocalyptic world, where most books have been destroyed. Having said that, they do find a copy of The da Vinci Code, and I struggle to believe that anyone would have kept a copy of that, over pretty much anything else. Anyway, such is the suspension of disbelief needed for these types of films.
The bible was banned and systematically destroyed because, as some believed, it was responsible for the war that had caused the apocalypse. This is pretty much all we get about the cause of the war, but religious books - actually, religion - does seem to cause people to want to kill others, irrespective of what their books actually say about killing other people. The book is sought by on person in particular - Carnegie - because "it is power" - and there is some truth in that. The bible can be a tool of worldly power, if people wish to use it in that way.
Carnegie actually then explains how it can give him power: "People will do what I tell them the book says" (roughly). Which sounds remarkably familiar - there are many who claim Christian leadership who are doing exactly the same thing, telling people what the bible says, and using its authority to support their position. What is more, it boosts their personal role, as the acceptable interpreter of the biblical words.
Carnegie understands very well the earthly power of a religious book and of those who can interpret it, and so speak with divine authority.
There is also Eli, who owns the book at the start of the film. Eli owns this copy, and reads it. He defends the book violently - which some would see as denying the values in the book. I don't see it in that way. Rather, he has the words of life, and he understands how important they are not just to him, but to everyone. He knows the value of the bible, and will defend that to the utmost. Until it comes to a point where it is the book or the life of his companion. At that point, he gives it up.
He then makes an interesting statement, that he has spent so long reading the book that he is in danger of missing what it says. To my mind, that is one of the most perfect lines in the film - the book is important, but not as a precious object, but because of what it says. The crucial matter was not the book itself. The crucial matter was the words in the book, and making sure that they are available, not in the control of a few. We are sometimes blase about having copies of the bible, without always actually wanting to read it.
And by read it, that means letting it speak and challenge.
Finally, when Carnegie finally gets hold of the book, he finds that it is in braille, because Eli is, apparently, blind. So he cannot, in fact, read it. He tries to force one of his people to read it for him, but she refuses, realising that he is incapable of wielding any power without her. There is something poignant in the fact that the person who wants to wield power with the bible finds that he cannot, in fact, read it, he needs one of those he despises to engage with it.
However, Eli finally gets to a place with a printing press. He has memorised the entire text, and so can dictate it, and allow it it be produced and printed, and so made available for everyone. Eli has internalised the words of the bible - as text initially, bit he also shows that, to an extent, he has internalised the meaning of it too - and it is this internalised version that is the key to making it available for all.
In the end, that is the story of this film: that the bible is important, it is powerful, but its power is not something that can be wielded like a weapon. It is powerful when it is made available to everyone, when it is made a part of us, when it is acted on. And when the meaning and interpretation is in the hands of a few, it is dangerous.