Vladimir Nabokov (1899-1977) is the case study where the synesthesia is documented in the first-person prose of one of the twentieth century's most precise literary stylists. In Speak, Memory (1947, revised 1966), Nabokov wrote at length about his grapheme-color synesthesia, in a passage that is one of the most frequently cited descriptions of the phenomenon in the entire clinical literature: "The long 'a' of the English alphabet… has for me the tint of weathered wood, but a French 'a' evokes polished ebony. This black group also includes hard 'g' (vulcanized rubber) and 'r' (a sooty rag being ripped). Oatmeal 'n,' noodle-limp 'l,' and the ivory-backed hand mirror of 'o' take care of the whites." The specificity of the descriptions — weathered wood versus polished ebony, oatmeal versus ivory — is not literary embellishment. It is the synesthetic experience being reported with the precision of someone who has had fifty years of practice describing it.
Nabokov's family was synesthetic in a pattern consistent with the substantial heritable component the contemporary research has since confirmed: his mother was a synesthete, his wife Véra was a synesthete, and his son Dmitri inherited the trait. Speak, Memory includes a scene in which Nabokov and his mother discovered that their grapheme-color palettes overlapped but did not match, and Nabokov as a child was intensely interested in the specific divergences — his mother's M was a different red from his M, and the mismatch was an early experience of the strange objectivity of the synesthetic fact.
What Nabokov's synesthesia produced, when pointed at the domain of writing, was a prose style characterized by the famously over-saturated, almost physical visual texture of individual words and sentences. Readers of Nabokov in any of his three working languages (Russian, English, French) have consistently described the experience of reading him as unusually specific at the word level — as if each word were being placed with an awareness of its color, its texture, its weight, and its physical fit against the words adjacent to it. That is, as it happens, exactly what a synesthete writing prose would experience. The sentences of Pale Fire, the butterfly passages of Speak, Memory, the texturally saturated prose of Ada or Ardor — these are the output of a cognitive architecture in which language itself carried visual and tactile information as primary perceptual content, and the task of writing was the task of arranging the visual-tactile content into sentences that did justice to the arrangement. His readers did not need to be synesthetes to feel the effect. They only needed to receive what the synesthete had translated into prose, and the prose was specific enough to carry the translation forward into minds that perceived letters as letters and nothing more.