Con Edison, the utility company, used to post signs at its New York City construction sites that said: "Dig We Must." The slogan was meant to excuse inconvenience, but it also captured the frenetic spirit of the Big Apple. The city is always being blasted, jackhammered and rebuilt.

New, taller skyscrapers seem to appear overnight. It's hard to believe there's anything left of the past.

But as writer Kevin Baker discovered, a remarkable amount of old New York is still with us.

Baker, 40, has published "Dreamland" (HarperCollins), an entertaining historical novel set in New York circa 1910. The story features real-life gangsters, Tammany bosses, labor organizers and Coney Island freaks, and is a vivid, sights-and-smells portrait of the period.

As he researched the book, Baker was amazed to find so many significant buildings, public spaces and other icons of the past still standing in lower Manhattan. He began exploring these sites on foot, then started taking friends along. Now his periodic walking tours have received enough word-of-mouth publicity to be something of an event among writers, history buffs and others who savor the fun of knocking around Gotham City. I joined him on a recent drizzly morning.

Note: We did a lot of walking. The Dutch got their $26 worth. As Baker told me, you forget how big New York is until you do it on foot. But the trip is easily broken up with stops for food or drink.

We started at Union Square. A park since 1811, it has seen countless political and labor rallies over the years. Baker directed my eye to the northeast corner of the square, where sits the New York Film Academy and Union Square Theater. This building was the fourth and last Tammany Hall, dedicated in 1929. Tammany, today synonymous with big-city bossism and political corruption, faded in the mid-20th century. But the power this Democratic machine wielded in the 19th century was such that it was able to hold the 1868 Democratic National Convention in its building, then nearby on East 14th Street. Its presidential candidate, one Horatio Seymour, was crushed by Ulysses S. Grant, but Tammany still called the shots.

We walked a few blocks south on Fourth Avenue to Cooper Union, built in 1859 by one of the country's truly remarkable entrepreneurs and philanthropists, Peter Cooper. The Union is still a fine (and free) engineering school. The school's Great Hall is where Abraham Lincoln and countless others have spoken to overflow crowds.

We walked west to Astor Place, where Joseph Papp established his Public Theatre in the exquisite former Public Library building. We continued west along Eighth Street, took a left on University Place, then walked through a gate on the right into the Washington Mews, a marvelously preserved 19th-century cobblestone street lined with two- and three-story houses.

The end of the street brought us to Fifth Avenue. We took a left and walked through Washington Square Park, in the heart of Greenwich Village. It really was a village in the 1830s, to which people escaped from the squalor and cholera of the city, then much farther downtown. We walked a couple of blocks, to Macdougal Street, one of the great Village thoroughfares, and stopped at Le Figaro, a classic coffeehouse.

We ambled back to Washington Square, taking a right on Washington Square South. We took a left on Greene Street.

At Greene and Washington Place stands the building, now owned by New York University, where the Triangle Shirtwaist Fire claimed 146 lives in 15 minutes on the Saturday afternoon of March 25, 1911. The company was on the eighth, ninth and 10th floors. The city's fire ladders only reached the sixth floor. The owners kept the stairway doors locked so the young Jewish and Italian seamstresses wouldn't steal small bits of cloth. The disaster led to the passage of 56 reform bills, and the building is the site of an annual labor rally.

We walked over to Broadway, crossed onto Great Jones Street, Baker's favorite New York street name, and took it to the Bowery.

Taking a right on Prince Street, we walked to the corner of Prince and Mott Street, where sits the old St. Patrick's Cathedral, which is surrounded by a small graveyard. Built in 1811, it was the seat of the Archdiocese of New York before the new St. Patrick's on Fifth Avenue was finished in 1879.

We walked south on Mott to Broome Street, and took a right. At Broome and Centre Market Place is the magnificent Old Police Station, a fascinating granite pile from which police commissioner Theodore Roosevelt reformed the department. We continued south to Hester Street, took a left and walked about eight blocks to Orchard Street and one of the most unusual of the city's museums. The Lower East Side Tenement Museum has a visitors center at 90 Orchard St. and an actual three-story, 1863 tenement building at 97 Orchard St.

The building was abandoned in 1935, when the landlord couldn't meet Mayor Fiorello LaGuardia's new law that every apartment have a bathroom. It stayed buttoned up for 50 years and was discovered intact in the mid-1980s. By finding relatives of residents and a couple of former residents, the museum staff has reassembled apartments as they existed at various times in the building's history. "There's nothing like it for understanding how the people lived," said Baker, whose heroine, Esther, lived in a tenement on Orchard Street.

We walked east to Essex Street, then south a block or two to Straus Square. Off the square are two buildings of note. The Seward Park Branch of the New York Public Library is where thousands of Jewish immigrant children lined up around the block to get books. Across the street is the six-story building that housed the Jewish Daily Forward, which once sold almost 250,000 papers a day in Yiddish. Famed editor Abraham Cahan told his readers how to be good Americans and ran a "Gallery of Missing Men" to shame men who had abandoned their families.

We headed west on Division Street and walked through Chinatown, with Baker pointing out Confucius Plaza and the Music Palace, which is the last Chinese-language theater in the city. We continued west to Centre Street, took a left and were soon at Federal Plaza and City Hall Park.

Baker drew our attention to the Tweed Courthouse, a pleasant Georgian edifice behind City Hall that generated an enormous amount in graft payments.

Built by notorious Tammany boss William Darcy Tweed, the courthouse went up in 1862-1870. The building originally was supposed to cost $250,000. Tweed brought it in for slightly more than $13 million, a staggering sum at the time, most of which was stolen from the project. Tweed, for example, bought enough carpet to lay a swath from New York to New Haven, Conn.

We walked south on Broadway, stopping at the Woolworth Building to see the ornate lobby for which the building has been dubbed "The Cathedral of Commerce."

We continued to St. Paul's Chapel, where George Washington worshiped on the day he became president, and Trinity Church, where Alexander Hamilton and Robert Fulton are buried.

We took a left on Wall Street and walked a couple of blocks. On the left is Federal Hall, the site of Washington's swearing-in in 1789.

Across at 23 Wall St. is the House of Morgan building, its granite face still pockmarked from an anarchist bombing in 1920. The apparent target of the bombers, J.P. Morgan Jr., was in Europe. But 33 others were killed and 400 injured.

Now just a couple of blocks from the tip of the island, we finished our trip with a walk down Broad Street, stopping at Fraunces Tavern, where Washington bade farewell to his officers; the Customs House across from Bowling Green; and Castle Clinton, a rebuilt fort that once was the city's immigration station and later the New York Aquarium. Now it's mostly known as the place to buy boat tickets for the Statue of Liberty and Ellis Island.

A 1980 Columbia University graduate who could still pass for a first-year grad student, Baker said he has studied New York because "You ought to know about the place you live."

He says New York has had only two great mayors, DeWitt Clinton and LaGuardia, and he is not a particular fan of the incumbent, Rudolph Giuliani. But reflecting on the riots and grifts of the past, he allows that New York today is safer and better-managed than it ever was. Which, if nothing else, makes the walk more appealing.