There is an explanation though you may not find it entirely convincing. The other letter Humbert receives that day is from John Farlow, his Ramsdale friend of sorts, the man in charge of his worldly affairs. Humbert would have given him his present New York address, and Lolita could have asked John if he had it. The problem is that then Farlow would have known where Lolita is and would not have asked Humbert to "produce" her. But probably one could think up an explanation for that, too. There is the possibility that Lolita had heard from Farlow years ago that Humbert was living in New York and that she had just looked up his address in a Manhattan telephone directory.

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