Upton Sinclair described the novel as "one of the half-dozen greatest novels of the world," and remarked that Hugo set forth the purpose of Les Misérables in the 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.
Towards the end of the novel, Hugo explains the work's overarching structure:
The book which the reader has before him at this moment is, from one end to the other, in its entirety and details ... a progress from evil to good, from injustice to justice, from falsehood to truth, from night to day, from appetite to conscience, from corruption to life; from bestiality to duty, from hell to heaven, from nothingness to God. The starting point: matter, destination: the soul. The hydra at the beginning, the angel at the end.
The novel contains various subplots, but the main thread is the story of ex-convict Jean Valjean, who becomes a force for good in the world but cannot escape his criminal past. The novel is divided into five volumes, each volume divided into several books, and subdivided into chapters, for a total of 48 books and 365 chapters. Each chapter is relatively short, commonly no longer than a few pages.
The novel as a whole is one of the longest ever written, with approximately 1,500 pages in unabridged English-language editions, and 1,900 pages in French. Hugo explained his ambitions for the novel to his Italian publisher:
I don't know whether it will be read by everyone, but it is meant for everyone. It addresses England as well as Spain, Italy as well as France, Germany as well as Ireland, the republics that harbour slaves as well as empires that have serfs. Social problems go beyond frontiers. Humankind's wounds, those huge sores that litter the world, do not stop at the blue and red lines drawn on maps. Wherever men go in ignorance or despair, wherever women sell themselves for bread, wherever children lack a book to learn from or a warm hearth, Les Miserables knocks at the door and says: "open up, I am here for you".
More than a quarter of the novel—by one count 955 of 2,783 pages—is devoted to essays that argue a moral point or display Hugo's encyclopedic knowledge, but do not advance the plot, nor even a subplot, a method Hugo used in such other works as The Hunchback of Notre Dame and Toilers of the Sea. One biographer noted that "the digressions of genius are easily pardoned". The topics Hugo addresses include cloistered religious orders, the construction of the Paris sewers, argot, and the street urchins of Paris. The one about convents he titles "Parenthesis" to alert the reader to its irrelevance to the story line. He devotes another 19 chapters to Waterloo, the battlefield Hugo visited in 1861 and where he finished writing the novel. It opens volume 2 with such a change of subject as to seem the beginning of an entirely different work. One critic has called this "the spiritual gateway" to the novel, as its chance encounter of Thénardier and Colonel Pontmercy foreshadows so many of the novel's encounters "blending chance and necessity", a "confrontation of heroism and villainy".
Even when not turning to other subjects outside his narrative, Hugo sometimes interrupts the straightforward recitation of events, his voice and control of the story line unconstrained by time and sequence. The novel opens with a statement about the bishop of Digne in 1815 and immediately shifts: "Although these details in no way essentially concern that which we have to tell..." Only after 14 chapters does Hugo pick up the opening thread again, "In the early days of the month of October, 1815...", to introduce Jean Valjean.