Gadda's novel may be closer to postmodernists like Pynchon than to modernists like Joyce. But Gadda wouldn't have cared; when he wrote this almost nobody in Italy knew about modernism vs. postmodernism. It might be described as a philosophical crime novel--but it isn't the usual crime novel. Basically it's a literary masteripiece which happens to be *also* a crime novel. In it you have everything you usually find in a "classical" whodunit: a victim, a detective, some suspects, police inquiry, and the culprit. But these things are no more than a pretext for such an immense writer like Gadda to talk about Fascist Italy and the city of Rome (Gadda was born in Milan, but he chose to move to Rome and knew the city and the surrounding area incredibly well). Then you have his gift for language, his corrosive irony, his restless intelligence, his deep understanding of the human mind (also with a lot of psychoanalytical insight). Plus a wealth of references to Italian and Latin literature (such as the Retalli family, whose names echo those of Aeneas' family in Virgil's Aeneid). Plus a wide knowledge of Italian geography and anthropology. Not bad for a man who had graduated in engineering!
The plot is often interrupted by lengthy descriptions. But those descriptions, which might seem pointless at a superficial reading, are the plot itself. In the novel, if you read it carefully, you are even told who really killed the rich signora of Via Merulana (btw, a street which really exists in Rome, though at n. 219 there is a shop, not a block of flats). But everything is shown obliquely, indirectly, through allusions and hints that you may easily miss on a hurried reading. I'd say that this is a novel that unfolds reading after reading--just like all real masterpieces.
I am not surprised Calvino extolled Gadda. Gadda is a slightly greater novelist than Calvino. Ehm, did I say "slightly"? I should have said "decidedly"! Obviously Calvino is one of the great ones... but good ol' uncle Carlo Emilio is one of the "greatest ones". I am afraid, though, that some of his greatness may get lost in translation, though he has been "rewritten" by such a fine translator as William Weaver.
It's a pity Gadda's other masterpiece, his essay Eros and Priapo, a bewildering but absolutely brilliant psychonalysis of Fascism (told in a baroque mix of styles), hasn't been translated into English. Hey, this ain't a perfect world, folks...