"From international bestseller and Booker Prize-nominee Siri Hustvedt comes a provocative novel about time, desire, memory and the imagination, Then tells the indelible story of a young Midwestern woman's fixation with her mysterious neighbor over the course of a threadbare year in 1970s New York" --
So there's a lot about the nature of memory, the way we unknowingly discard large amounts of information and retrospectively rewrite other experiences to suit the patterns we expect to find, and the way incidents move up and down in the scale of importance in unpredictable ways. It soon becomes clear that neither the diary nor the narrator's memory is entirely trustworthy, but as well as upsetting our preconceptions about whether it's possible to construct an authoritative version of past events, Hustvedt also has a lot of fun playing with our assumptions about how much her fictional S.H. (decoded for us variously as "Standard Hero", "Sherlock Holmes", etc.) can be identified with the author, throwing in a baffling mixture of real and fictional biographical details cunningly designed to prevent us from settling on either side of the fence.
Thrown into the mixture is the narrator's encounter with the papers of the then-forgotten dadaist, Baroness Elsa von Freytag-Loringhoven (1874-1927; she's now become famous as the probable real author of the celebrated conceptual artwork Fountain previously claimed by Marcel Duchamp). The bad Baroness's ribald anger gives S.H. a kind of virtual outlet for her frustration at the dismissive way she herself is often treated by a world that doesn't really expect blonde young women to have an opinion about Wittgenstein.
And of course this is also a very engaging novel about what it's like to be a young woman setting out with high expectations into the exciting world of New York in 1978. Making friends, partying, running out of money, going hungry, getting into trouble and out of it, making up stories about strangers and then discovering that the truth is both stranger than you imagined and more banal, deciding whether to put up with casual sexism or fight back against it, and all the rest of the weird world of being 23.