MOLTO PIANO.(Whitney Museum of American Art)

Renzo Piano comes from a family of builders in Genoa, and his firm is called Renzo Piano Building Workshop, as if he weren't a superstar architect but just your friendly neighborhood problem solver. Unlike most other architectural stars, Piano has no signature style. Instead, his work is characterized by a genius for balance and context, an ability to establish inventive correspondences between his buildings and those that surround them. Until now, New York hasn't had a building by Piano, but his expansion of the Morgan Library, which has just reopened as the Morgan Library & Museum, is the beachhead for what may become a significant presence in the city. A skyscraper that will serve as the new headquarters for the Times is nearing completion, and Piano's addition to the Whitney Museum of American Art is expected to begin construction within the next two years.

Piano's achievement at the Morgan is to have created a satisfying composition out of three not particularly compatible landmark structures. The old Morgan buildings are awkwardly placed around the intersection of Thirty-sixth Street and Madison Avenue. The best of them, a dazzling Renaissance palazzo that Charles F. McKim designed as J. P. Morgan's personal library in 1906, is tucked away on Thirty-sixth Street. A dull, classical annex next to it, erected by Benjamin W. Morris when the library was opened to the public, in the nineteen-twenties, has the choice location, on the corner, and north of this is a staid nineteenth-century brownstone. The challenge was to link these disparate buildings while also increasing the institution's space by half. Piano's solution is simultaneously subtle and radical. To avoid overwhelming the existing structures, he hid most of the new space underground, sinking storage vaults and a concert hall four stories into the bedrock. At ground level, an elegant glass-and-steel structure weaves between the older buildings and meets the street in two rectilinear facades, one on Madison Avenue and the other on Thirty-sixth Street.

Piano has reorganized everything so that the Madison Avenue facade is the new entrance, but it is from Thirty-sixth Street that you see most clearly how he has both preserved the Morgan and transformed it. The two limestone buildings, McKim's palazzo and Morris's annex, formerly tethered by a dreary connecting walkway, had an uncomfortable relationship with each other. Piano replaced the walkway with a compact steel cube painted a color that echoes the limestone. (Inside it is a gallery of medieval religious objects.) The cube is divided into eight ridged panels, to give it scale and texture, and it provides a firm counterweight to the mass of buildings on either side. It suggests that Piano is less concerned with form-making than he is with balancing masses, materials, and scales. I'd always found Morris's annex bland, but now it seems crisply defined, while McKim's opulent palazzo has acquired a new solidity, at no cost to its elegance.

Piano does something similar on Madison Avenue, but it is less effective. The limestone buildings on Thirty-sixth Street always had an innate competitive tension, but the austere side wall of the Morris annex and the grand old brownstone are two structures with little to say to each other, and they are separated by a wider gap. Piano fills the gap with an elongated version of his steel cube--which houses a new reading room--but the proportions aren't as enticing as they are on Thirty-sixth Street. Below the reading room, the new entrance consists of glass doors stretching across the facade, an improvement over the cramped front door in the old annex, but still a little conventional.

Inside, however, the ingenuity of Piano's conception becomes gloriously apparent. He has essentially created a piazza: a fifty-two-foot-high, glass-covered atrium in the middle of the complex, with access to all parts of the institution, new and old. Too often, when exterior walls of old buildings get turned into interior elements of stylish new spaces, they take on the precious air of stage sets. ...