What Mira said. Impressive indeed. The narrative voice, with its seeming distance and impassivity, works so well. I like the absence of names and the power of the various paradoxes and imprecisions (numbers with no answers, the perfect interchangability of One leans over to tuck a loose strand of hair behind the other's ear, the description of the professors' non-verbal relationship of the previous year).
Other things I love: --The traces of the Founders, so chracteristic and revealing --The castle, perhaps the most fascinating character of all (Stone hums its sorrow. . .) --That wonderful sex/love/sensation paragraph, with both women presented as sort of their own essences (great line: three involuntary gasps: wholeness; nine firm tongue-strokes (nine letters in please, yes): completion)
The prose reads like the uncertainty principle put into words, if that makes any sense.
(one small mechanical nitpick -- in paragraph seven, "unaccessable" should be "inaccessible," unless you want the other version for rhetorical purposes)