"I came across this passage in The Waste-Land"

 The Waste Land is a famous Modernist poem by T.S. Eliot, first published in 1922. The whole text of the poem, with a useful selection of notes, can be read online here. Some commentators have suggested that Tartt's reference to this poem is nothing more than showing off; they are unable to see the point. However, a closer reading of this passage makes the reason for its presence obvious. Henry is moved to discuss Elizabeth and Leicester whilst rowing a boat in the lake at Francis's house; it seems clear that this activity reminds him of the scene in The Waste Land in which Elizabeth and Leicester are themselves in a rowing boat, a connection made explicit by Richard's quotation. This scene could also be read as a covert hint at the secret understanding between Henry and Camilla.