The Real Life of Sebastian Knight: Very like a masterpiece?

I've been rereading Nabokov's The Real Life of Sebastian Knight (published 1941, the first novel he wrote in English). My copy is a Penguin reprint dating from 1971 - that handsome set with the Nabokov signature aslant the cover - and it would be around that date that I first read it. If I've read it again since, it would have been at least 20 years ago, so I was glad to find how much of The Real Life I remembered - scenes, phrases, images, the overall shape...
 It's a quite extraordinary book, this one, and seems all the more so on rereading. Ostensibly an unnamed narrator's (or rather named only as V) account of the life and works of his adored half-brother, the distinguished Russian-born novelist Sebastian Knight, The Real Life soon has the alert reader questioning what exactly is going on here. Are V and SK 'really' separate entities, or is one the creation of the other - and if so, which way round? Of course both are the creations of VN and have their being in the novel The Real Life, which itself contains the novels of SK, an abysmally bad biography of SK by one Goodman, and (by way of SK's autobiographical Lost Property) the ghostly presence of VN's yet to be written Speak Memory - not to mention the later novels The Real Life prefigures, notably Pale Fire and Transparent Things.
 All of which makes The Real Life sound like some tiresome postmodernist exercise in metametafiction - but it is no such thing. Nabokov's grace, wit and style, working through the medium of the somewhat plodding V and the brilliant SK, keep the narrative shimmering with life. As the story proceeds - in a series of Knight's moves, naturally (the novel is full of chess allusions) - it becomes a thoroughly enjoyable page-turner, albeit a page-turner of exceptional literary quality. I had a lurking doubt that this reread might prove a disappointment - but in the event I was impressed anew by what now seems to me very like a masterpiece.