You know how people talk about books that "changed my life"? Well, I don't think books can change your life (at least not in my case), but I think they can help you change your life (in other words, you change your life, not the book -- ok, pedantic point, but still, wanted to be clear on that....)
Well, this book, along with Buzzati's Il deserto dei Tartari helped me change my life (actually, I have 2 non-fiction examples that did the same, albeit in different ways: Lewis Hyde's The Gift, and Norman O Brown's Life Against Death).
How did this book help change my life? By showing me, in stark color, myself. It didn't show me something I didn't already know. What it did was help me to see what I already knew rationally in another, more emotional way. It showed me the tortured mechanisms of my mind for what they are: bullshit wankitude. It showed me what happens when you don't see things for what they are (Buzzati's book did the same): you don't live.

A note on the style: I enjoyed the Antonioni-esque usage of landscape and weather as a backdrop to the psychological drama.

Interesting trivia point: on its initial publication in 1898 this book had no success, and was not mentioned (either favorably or unfavorably) by any critic. 25 years later, James Joyce convinced Svevo to release a 2nd edition. Thanks James!