From the author of the Booker Prize-winning Moon Tiger (1988), etc.: a serious, self-involved meditation on transience and immutability, with a map of London--present and past--laid on top. Matthew Halland, an architect undergoing a period of low- level mourning over the end of his marriage, roams about London visiting construction sites and entertaining his eight-year-old daughter. The child's innocently deep questions--''Why are there so many stars?'' ``Is there always another day?''--set off riff- like internal monologues, during which Matthew ponders the death of love and, really, the whole point of going on. A moral dilemma arises when he's pursued by a crooked real-estate tycoon, and a hopeful little light gets switched on when he meets Sarah Bridges in a sandwich shop. Their love blossoms slowly, tenuously, but still seems a reassuring sign, for when one feeling dies, apparently a new one can be born. Meanwhile, Lively underscores the story with images of London's myriad neighborhoods, including fragments of the city as seen by a WW II fire warden, a street urchin, and a Victorian paleontologist--all of which are meant to suggest the presentness of the past, or ``the immortality of the whole ensured by the transience of the many.'' Obviously thinky--indeed, a tad too introspective--but Lively is still a searching, gifted writer, mistress of a tightly reined-in sentimentality that will hit any reader who's experienced a funk such as Matthew Halland's.