How do the indications of social injustice in Les Misérables relate to the book's title?

To draw another analogy, Victor Hugo is the French version of the English Charles Dickens.  Contemporaries in time, both men perceived the tremendous gap between the upper classes and those in the lower.  While Dickens described society as a "prison" in which few, if any, could rise above their position in society, Hugo writes of Jean Valjean, condemned unjustly to prison, who escapes the physical prison, but is unable to escape the "prison" of always having been a criminal. 

For one thing, he must carry a yellow card identifying himself as a criminal.  As a result, he cannot find work, and, in desperation--a state which precipitated his first crime--he steals the candle stands from the bishop.  It is only because of the charity of the bishop who tells the gendarmes that Valjean was given these valuables by him that Valjean, a "miserable," a chance in life.  And, with this chance, Valjean redeems himself in many ways, practicing the same charity towards other "miserables" such as Fantine and Collette. Nevertheless, he remains condemned by society and is constantly pursued by Inspector Javert until the end of Javert's life.

Similarly, in Dickens's "Great Expectations" in which Magwitch, an unfortunate "wretch"--a "miserable" who also has stolen to keep from starving--escapes from the prison ship on which he has been sentenced after his complicity with a gentleman who has taken advantage of him and strives to redeem himself in New South Wales.  After he becomes a wealthy sheep farmer, he acts as Pip's benefactor, but he is still condemned by the "prison" of society, and cannot escape the fate of England's laws.  Thus, both Dickens and Hugo write of the miserable fate of the poor, who albeit good in soul, cannot redeem themselves in society.