Sylvia Plath's entire body of work is, in a sense, an exploration of mental states, but 'The Bell Jar' is the most direct. It's a semi-autobiographical novel that chronicles Esther Greenwood's descent into depression and her experiences with electroshock therapy. The prose is terrifyingly lucid, capturing that dissociative feeling where the world just... grays out. It's less about diagnosing a condition and more about conveying the texture of the experience from the inside.
Her poetry, especially the collection 'Ariel', is where the raw, unfiltered intensity lies. Poems like 'Lady Lazarus' and 'Daddy' aren't clinical studies; they're volcanic eruptions of anguish, fury, and a fragmented self. The mental health themes there are inextricable from her poetic voice—the imagery is sharp, sometimes violent, and breathtakingly honest. Reading 'Ariel' feels like holding a live wire.
I'd argue her earlier novel, 'The Bell Jar', gets the spotlight, but the poetry offers the deeper, more complex anatomy of a psyche under extreme pressure.