Zeno's Conscience' is such a unique blend of introspection, dark humor, and psychological depth—it's hard to find something exactly like it, but a few books come close in spirit. If you love the way Svevo dissects human folly with a mix of irony and pathos, you might enjoy 'The Confessions of Zeno' vibes in 'The Notebooks of Malte Laurids Brigge' by Rainer Maria Rilke. Both protagonists are hyper-aware of their own flaws, though Rilke leans more poetic.
Another contender is 'Death of a Nobody' by Jules Romains, which has that same meandering, almost stream-of-consciousness style where the protagonist obsesses over his own irrelevance. For a modern twist, 'The Loser' by Thomas Bernhard delivers relentless self-scrutiny with a biting, circular narrative. None of these are perfect matches, but they all scratch that itch of existential comedy wrapped in personal failure.