Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
The main character in VN's novel Bend Sinister (1947), the philosopher Adam Krug is the author of the Komparatiwn Stuhdar en Sophistat tuen Pekrekh (The Philosophy of Sin in the American edition), a book that was translated into English by Krug's friend Ember: