Humbert uses the term 'liberal libations'. Such alliteration could sound overly stylised but it doesn't here because the voice of his character is so strong. The alliteration is not a sign of the author breaking voice, it is an erudite and bookish way of saying drunk, and exactly the sort of thing we can imagine Humbert saying. And because alliteration like this jumps out of the page, when it is effecive it is actually remarkably effective, as it is here:
Such-like fragrant vagabond thoughts have been always a solace to me in times of unusal stress, and only when, despite liberal libations, I felt fairly numbed by the endless night, did I think of driving back to the motel.