Now is a good time to look at the New York Public Library, on Fifth Avenue between 40th and 42nd streets. The library has announced an exterior restoration to be completed by 2011, the 100th anniversary of its opening. The architects of the library, Thomas Hastings and John Carrère, worked closely with the library’s director, John Shaw Billings, in creating a model of efficiency, rational planning, and beauty. In recent years, we have seen a magnificent renovation of the library’s interiors. The Main Reading Room, especially, is a glorious space, all the more so for being cleaned up, and for the outstanding way that architects and library planners incorporated new technology into the space. At a time when London got a new British Library and Paris a new Bibliothèque Nationale, New York did the right thing in managing growth and change by retaining the central branch as the system’s core arts and humanities library while dispersing other departments to other locations. This has allowed the library to keep the building as its symbolic center while ensuring that it is not overburdened.

The exterior now awaits its due. The Vermont marble façades are in a state of advanced disrepair, and the library has had to place netting over parts of them for fear of falling stone. I have a feeling that when the work is done, it’s going to be a profound thing. The marble is the kind that’s on the outside of the New York Stock Exchange on Broad Street, though you’d never know it. It’s a blue-veined creamy white marble — it looks like cheese. The Stock Exchange, by contrast, has always had the funds to keep its stone in trim. At the library, be prepared for a surprise, if not a shock.

Work begins this spring on the rear of the building, facing Bryant Park. This is the “striped” façade, with vertical strips providing external identification of the book stacks within. The arcaded upper section is the west wall of the Main Reading Room. Notice the odd little doors set between the windows. People wonder what these were or are for. The best I can make out is that perhaps Carrère & Hastings thought it possible that the stacks would one day be expanded via a low addition onto the back of the building. The doors from the Main Reading Room might then provide access to rooftop space on the addition, functioning as the kind of outdoor reading room that the library’s rear terrace has at various times. That rear terrace, which also functions as the east end of Bryant Park, wraps around to Fifth Avenue, where the library is set back from the street behind a generous elevated plaza. It’s the sort of thing that if it were done with a modern skyscraper would be condemned as bad planning, the sort that cuts the building off from the street life around it. In fact, the local street life fairly swarms over the library’s terrace. The grand stairway is one of the most popular spots in midtown, and the terrace acts as an extension of Bryant Park.

The skyscraper at 500 Fifth Ave. — across the street at the northwest corner of Fifth and 42nd — dates from 1931. Shreve, Lamb & Harmon designed it just before, and it was built concurrently with, the same architects’ Empire State Building. It’s nice that it should be right across from the library, since Richmond Shreve and William Lamb had worked for Carrère & Hastings, and the firm of Shreve, Lamb & Blake took co-credit with Carrère & Hastings for the design of the Standard Oil Building at 26 Broadway, one of the greatest skyscrapers in New York. Shreve and Lamb were office-building specialists, and when they added Arthur Loomis Harmon, who had designed the influential Shelton Towers Hotel, now the New York Marriott East Side (1924) on Lexington Avenue at 48th Street, they got commissions for extremely tall buildings.

When it was built, 500 Fifth was the sixth-tallest building in the city; it is now 36th, at 697 feet. It has the classic massing of its era. Between 1916 and 1961, zoning rules mandated the stepped silhouette of tall buildings in New York. The best skyscraper architects used the rules for dramatic effect. If architecture is frozen music, then the stepping, projecting, and receding planes of 500 Fifth are like the interplay of horns in a swing band.

Notice how clearly we can see the difference in zoning between Fifth Avenue and 42nd Street. You could rise higher without a setback on the crosstown street than on the avenue. That’s why this building is especially nice when viewed from the south. Because of the low-rise library, we have an unobstructed view of 500 Fifth, and can see its mountainous ascent. We can also see sets of masses stepping back in two directions: from east to west and from south to north. And we can see the dramatic, sheer rise of the building’s tower, with its brick-faced piers and vertical window sections creating a bold striped effect. It’s hard not to think the architects borrowed from the rear of the library, perhaps in homage to their mentors.