Joseph Conrad’s novella Heart of Darkness had a special significance for T.S. Eliot. He made references to it in The Waste Land, as described in this note to the essay Notes on the Publishing History and Text of The Waste Land (1964, reproduced in The Waste Land Casebook Series):

In the first of the published letters between Pound and Eliot on the poem, Pound said, ‘I doubt if Conrad is weighty enough to stand the citation’ (Letters of Ezra Pound, p. 169). Hugh Kenner (p. 145) learned from Eliot that Pound referred to Eliot’s quotation of ‘The horror! The horror!’ from Heart of Darkness. As Pound suggested, Eliot removed the quotation. But Pound apparently was unaware that the words in lines 268-70 of The Waste Land were derived from the first page of Heart of Darkness (pointed out by Kenner, p. 145, and rediscovered by John Frederick Nims, ‘Greatness in Moderation’, in Saturday Review, 19 Oct 1963, p. 26), and that various passages in the poem concerning the Thames are strongly reminiscent of the first few pages of Conrad’s novel.

The lines from the Waste Land referred to above are the following from part III, the Fire Sermon:

Red sails
The barges drift
With the turning tide

This would appear to be inspired by this passage, from the second paragraph on page one of Heart of Darkness (although the significance of the reference is not clear):

In the offing the sea and the sky were welded together without a joint, and in the luminous space the tanned sails of the barges drifting up with the tide seemed to stand still in red clusters of canvas sharply peaked, with gleams of varnished spirits.

Eliot also quotes from Hearts of Darkness on the title page of his poem the Hollow Men: ‘Mistah Kurtz – he dead’. (In a nod to this quotation, Coppola has Kurtz reading the Hollow Men in one of the final scenes of Apocalypse Now).

What did Heart of Darkness mean to Eliot, and why the recurring quotations? C. Day Lewis hints at the connection in his essay A Hope for Poetry (1934, reprinted in The Waste Land Casebook Series):

[The Waste Land] makes us aware of the nervous exhaustion, the mental disintegration, the exaggerated self-consciousness, the boredom, the pathetic gropings after the fragments of a shattered faith – all those symptoms of the psychic disease which ravaged Europe as mercilessly as the Spanish influenza. But in doing so it enlarged our conception of the field of poetic activity: as Eliot himself has said; ‘the essential advantage for a poet is not to have a beautiful world with which to deal; it is to be able to see beneath both beauty and ugliness; to see the boredom, and the horror, and the glory’.