"Percy Elphinstone" is the supposed author of A Vagabond in Italy, one of the "corruscating trifles" that Humbert encounters in the prison library, and is also said to be the author of Venice Revisited,  Boston, 1868.  Nabokov wrote to Appel that "A Percy Elphinstone did write A Vagabond in Italy which I found in a hospital library, the nearest thing to a prison library. The "Elph-in-stone" is wrong. It is my own random recollection of Percy, a happier vagabond than my man." Appel somewhat misleadingly quoted the first sentence, but not the other two, in The Annotated Lolita. Nabokov's letter suggests that he invented the last name, but not the first name. Efforts to locate Percy Elphinstone have proved unavailing.  If we assume that the last name is an invention, a possible candidate is Percival Pollard, author of Vagabond Journeys: The Human Comedy at Home and Abroad (NY: Seale, 1911), ( https://archive.org/stream/vagabondjourneys00pollrich#page/n9/mode/2up ) which includes a chapter titled "Vandalism in Modern Florence."  Nabokov's recollection of the title and of the destinations of Percy's travels may have been slightly inaccurate, as his own remark about the "random recollection" suggests. The subtitle, concerning the "human comedy," may also explain why Nabokov considered this Percy "a happier vagabond" than HH.

- Simon Stern