Like any good exhibition, "New York Eats Out," based on the New York Public Library's Buttolph Menu Collection, raises as many questions as it answers: Was "Calves' Head, Brain Sauce" popular with the women who ordered from the Astor House's 1843 "Ladies Ordinary" menu, or was it trumped by the more dainty sounding "Small Birds, Italian Style"? What was "Bulgarzoon Scientifically Fermented Milk," and why did Childs restaurant advertise it on the cover of its 1940 menu? And what health codes governed the street-cart clam vendors who once crowded the Italian section of Harlem?

It's hard to imagine what the New York Public Library thought when Miss Frank Buttolph wrote to them in 1899, asking if they would be interested in acquiring menus. Did the library director's "yes" come more from a desire to humor this eccentric figure, or from a true interest in preserving these usually ephemeral slices of social history for future scholars? Either way, the end result was an invaluable collection of more than 25,000 menus that Miss Buttolph collected for some 25 years, until her death in 1924.

The value of this holding, haphazardly added to over the years, was recognized by current library president Paul LeClerc in 1999-a coincidental 100 years after Buttolph's first inquiry. LeClerc approached author and New York Times restaurant critic William Grimes about curating an exhibit based on the collection, and the result is an intriguing, often quirky look at a city's social history through its obsession with dining.

As his starting point, Grimes chose the venerable Delmonico's, New York's first purveyor of grand cuisine — along with a heady dose of entertainment. The history of the Delmonico dynasty includes many elements familiar to modern restaurateurs: the opening of branches in other parts of the city, moving several times due to real estate concerns, and staff defections. Ultimately this doyenne succumbed to financial realities affecting the restaurant industry in the 1920s, most notably Prohibition.

Delmonico's and its elegant counterparts such as Sherry's, Café Martin, and The Waldorf-Astoria are just part of the varied dining scene Grimes highlights. New Yorkers of all walks have always taken an inordinate number of meals outside the home, and venues from street carts and stand-up oyster cellars to ethnic eateries and low-priced chains served New Yorkers in need of fast, cheap eats. In their heyday, Swiss-invented automats alone served a startling 800,000 meals a day to New Yorkers in surroundings that may look sterile and utilitarian by today's standards, but which diners of the time found healthful and modern. Many New Yorkers still remember the lone Horn & Hardart outpost on 42nd Street and Third Avenue (it closed in 1992), but at one time some 50 branches dotted the city.

A treasure of striking menus come from the city's 1939 World's Fair. A bewildering collection of 80 food outlets populated the fair, many connected with the national pavilions and serving authentic (to varying degrees) international cuisine. In keeping with the fair's progressive, optimistic "The World of Tomorrow" theme, a children's menu entices us with "Irradiated Vitamin D Milk," and a menu from the U.S.S.R. pavilion sternly advises that "Our employees are adequately compensated. Following Soviet custom please do not tip."

Grimes ends the exhibition with an overview of fine dining through the '50s and '60s, culminating with the storied history of Windows on the World. Like the World Trade Center itself, the restaurant was a daringly modern endeavor and one of the first truly contemporary restaurants.

Curator Grimes still has questions of his own after nearly two years of sorting through and compiling this exhibit. In particular, he's struck by the exhaustive list of dishes contained on many of the menus. "Were all of these items really available on a daily basis, before widespread refrigeration? Or was it more of a theoretical list of possibilities that would change from day to day?" Despite historical and fictional accounts of restaurant meals, Grimes also wonders about the number of different courses diners would have typically chosen to comprise their meal. "Although I can say men always finished with cheese, coffee and cigars; I think dessert was aimed almost exclusively at female diners."

On the subject of the mysterious Miss Buttolph, Grimes thinks we may have 21st century counterparts, some of whom can be found enthusiastically bidding for historical menus on eBay — a resource Buttolph would certainly have envied.

"New York Eats Out" runs through July 12. Admission is free.
The New York Public Library, Fifth Avenue and 42nd Street
212/869-8089, www.nypl.org.