''In an era of inept and ignorant imitations, whose piped-in background music has hypnotized innocent readers into fearing literality's salutary jolt, some reviewers were upset by the humble fidelity of my version...'' Such was Vladimir Nabokov's response to the storm of controversy aroused by the first edition of his literal translation of ''Eugene Onegin''. This bold rendering of the Russian masterpiece, together with Nabokov's detailed and witty commentary, is itself a work of enduring literary interest, and reflects a lifelong admiration for Pushkin on the part of one of this century's most brilliant stylists.