The New York Public Library, with its headquarters at Fifth Avenue and 42nd Street, encompasses four major research libraries and 87 branches, with a total of 20 million books, 50 million cataloged items and a growing demand in this recession for loaner laptops and other free services. Only the Library of Congress and the British Library are larger. But even the Fifth Avenue landmark by itself is a marvel of big numbers. It is undergoing a $1.2 billion makeover in preparation for its 100th birthday. Built from 1899 to 1911, it cost $9 million, contains 530,000 cubic feet of white Vermont marble and 125 miles of shelving, and opened with an inventory of one million items. Here are some highlights of this fabled institution.

ANSWER ZONE The library began its telephone reference service in 1968; it is available online or by calling 917-ASK-NYPL (917-275-6975).

● How old is the moon? 4.72 billion years.

● Where was the toothbrush invented? China, in 1498.

● How often does the service receive questions? Every 10 seconds.

SADDEST ITEM In August 1820, a dying John Keats wrote a letter to his soul mate, Fanny Brawne, that begins with a fond salutation — “My dearest girl” — but goes quickly downhill. “I am glad there is such a thing as the grave,” he wrote. “The world is too brutal for me.” It ends with a romantic flourish: “I wish that I was either in your arms full of faith or that a thunderbolt would strike me.”

Image HEAVIEST BOOK A tie between “Michelangelo: La Dotta Mano,” a handmade coffee-table-size trophy, and John James Audubon's 1830s first edition of “Birds of America,” with its life-size raptors and flamingos. Both weigh about 61 pounds. Credit... Fred R. Conrad/The New York Times

MOST FAITHFUL CUSTOMER Norbert Pearlroth, the head researcher for “Ripley's Believe It or Not,” visited the library almost daily. Although he wrote about the incredible, his own routine was anything but: He sat at the same table for 52 years, from 1923 to 1975.