The driving question behind any dissection is: "What is the purpose of each part, and how does it contribute to the whole?" This is the question I will attempt to answer for every chapter, beginning first with the Foreword.
Dr. John Ray, Jr.'s Foreword prepares us to grapple with the difficult subject of Lolita, instructing us on how to proceed and easing us into the bubbling waters of Humbert Humbert's mind.
Ray presents himself as a friend and relation of H. H.'s lawyer, who requested he edit the manuscript for publication, obscuring any details that might reveal the true identities of its characters. Yet he purposely rustles the veil hiding Nabokov the novelist behind Ray the editor. Although Ray is ostensibly a psychologist, his foreword expresses the reservations of the novelist - even sounding peremptorily defensive ("a great work of art is of course always original, and thus by its very nature should come as a more or less shocking surprise.") - more than those of a doctor, "suggesting that the mask [hiding Nabokov the novelist] has not remained totally in place" (Appel). As H. H. demonstrates (like in many of Nabokov's works), "illusions are realities in their ability to destroy us" (Appel). So let's not take any division between reality and fiction too literally.
Ray emphasizes that we should not exalt H. H., and that this work is not an excuse for his actions. "A desperate honesty... does not absolve him... But how magically his singing violin can conjure up a tendresse, a compassion for Lolita that makes us entranced with the book while abhorring its author!" He concludes by urging us to raise "a better generation in a safer world." His moralizing outside view anticipates the judgment of the reader, attempting to prevent the reader's alienation from H. H. the narrator. This is absolutely necessary, because if we dove straight away into the mind of Humbert Humbert, we would find the water unbearably hot. Ray's Foreword prepares us a little for the plunge.
One other very important accomplishment of the Foreword is that through Ray, Nabokov reveals that all three main characters (Humbert, Lolita, and Quilty) are now deceased. This anachronistic reveal of their fates violates the reader's expectation of chronology and surprise, suggesting that the point is not their destinations, but how they got there.
It is also worth noting that Nabokov notoriously hated psychoanalysis, so having a psychologist narrate the Foreword is a playful warning not to overanalyze.
Stylistically, there is not much worth noting in the Foreword, save for some lovely phrases ("a tempest in a test tube") and funny, thought-provoking asides ("Do the Senses make Sense?"). It is primarily functional, as it allows Nabokov to speak a little more directly to us as a novelist, instructing us on how to proceed with this very difficult, complex, rich, and gorgeous work of literature.