Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
In Canto Four of his poem John Shade (the poet in VN’s novel Pale Fire, 1962) says that the title of his first book (free verse) was Dim Gulf:
Dim Gulf was my first book (free verse); Night Rote
Came next; then Hebe's Cup, my final float
In that damp carnival, for now I term
Everything "Poems," and no longer squirm.