I had to pare down the original question. Hugo's novel is rooted in the idea of exposing injustice and seeking to bring attention to the fact that it is a condition which must change. I think that one of the most effective quotes about injustice can be seen in the work's Preface:
SO long as there shall exist, by reason of law and custom, a social condemnation, which, in the face of civilization, artificially creates hells on earth, and complicates a destiny that is divine, with human fatality; so long as the three problems of the age—the degradation of man by poverty, the ruin of women by starvation, and the dwarfing of childhood by physical and spiritual night—are not solved; so long as, in certain regions, social asphyxia shall be possible; in other words, and from a yet more extended point of view, so long as ignorance and misery remain on earth, books like this cannot be useless.
Consider that this quote from the Preface actually encompasses so much of what is explored in the novel. The idea of "hell on earth" can describe the condition of Fantine and others like her who must endure severe social and economic degradation without any sort of care from those in the position of power. In the fight of the revolutionaries exists a desire to alleviate the "social asphyxia" that Hugo felt had a hold on France at the time. The "degradation of man by poverty" is Valjean's life story. The relief from "ignorance and misery" is one of the primary motivations of Hugo's work and in doing so, one can see how the drive to remove injustice from that which is becomes one of the fundamental points made in the quote that underscores the entire work . It is the desire to make what is into what can be that ensures that "books like this cannot be useless." In this is a great and effective quote about the nature of injustice.