This is a highly singular book, not only for the language employed (a blend of Romanesco and Italian) and its style, but also for its narrative, which, while framed within a typical detective plot, seems to serve as an occasion to reconstruct the social milieus of 1920s Rome.
Two crimes occur on Via Merulana in Rome, in an apartment building of the Roman upper-middle class: several pieces of jewelry are stolen and, the following day, a woman is murdered.
Thus unfolds the judicial inquiry, conducted partly by the police and partly by the Carabinieri.
This investigation serves as the framework within which a series of social and individual portraits are developed.
The settings consist of the Roman bourgeoisie and apartment-building life, the world of the police with its bureaucratic mechanisms and petty rivalries, and the Roman periphery with its poverty and desolation.
The characters are embodied by Inspector Don Ciccio, the married couple—one of whom is the murder victim—the women of the Roman outskirts, as well as numerous minor figures.
Beyond their differences, the author brings to the fore the weary resignation of life, devoid of novelty or change, the paltriness of human feeling (the primacy of money and petty jealousies), and a pervasive mediocrity: all tempered by a gentle irony and an overt understanding of life's tribulations.
The language represents one of the novel’s most significant features: it is characterized above all by the use of Romanesco and, consequently, by an attempt to transcend the artificiality of literary prose, which renders the dialogues and conversations remarkably vivid.
From this perspective, the lengthy police interrogation of a woman (Ines) is magnificent.
Outside the dialogues, the author employs an invented language, rich in Romanesco terms, standard (even classical) Italian, and, in my view, neologisms as well.
A D'Annunzian pursuit of sonority and effect emerges, which at times weighs the prose down and contributes to a sense of artificiality and formalism.
As written in the Enciclopedia della Repubblica,

Gadda's work stems from an extremely fraught and violent relationship with reality, conceived as intimately convulsed, deeply and irremediably misshapen, disordered, upheaved, so much so that to it one can apply only a word that rivals its deformity, urging it to reveal itself fully.... the blending of dialects...
as a divergence from the more orderly and mystified forms of language; then the metaphorical strainings, pushed to extreme tensions, of technical jargons; the intricately branched and labyrinthine syntax, through which the intention to thoroughly corrode the customary organization of things is manifested

That Awful Mess on Via Merulana