New York -- When E. L. Doctorow writes about the past, sometimes he uses his memory -- flawed though it may be. "I have a terrible memory," he told an interviewer once, though it served him well enough to write such evocative novels as "World's Fair" and "Billy Bathgate," both set in the 1930s in his native New York.

Other times, it's images he uses. For instance, his rambling old house in New Rochelle, N.Y., got Mr. Doctorow thinking about the history of the place and then beginning a novel set in the early 1900s. That was "Ragtime," his breakthrough 1975 book, which won him not only great critical acclaim but also wide popular success.

Perhaps no other contemporary author uses the past to such great effect in fiction as does Edgar Lawrence Doctorow.

His novels are not only set in the past, they also embellish it. Historical figures, such as J. P. Morgan in "Ragtime" and Dutch Schultz in "Billy Bathgate," become major characters in his fiction. His novels have been called historical fiction, but they really are reworkings of history through his fertile imagination.

A few years ago, Mr. Doctorow started thinking about the huge reservoir that used to stand at 42nd Street and Fifth Avenue in New York. He thought about New York in the 1870s -- a city on the move, relentlessly modern, embracing the latest innovations of the Industrial Revolution. He pictured it to be a time both of fabulous wealth and great poverty, of exciting possibilities and also corruption and social dislocation.

Out of these imaginings came his sixth novel, "The Waterworks," which will be released this month by Random House. Once again, Mr. Doctorow has been drawn back to the past, and once again he has richly reinvented it.

The vividness of Mr. Doctorow's writing, his attention to detail, has won him many admirers among other writers. One is William Kennedy, the author of the Pulitzer Prize-winning novel "Ironweed," whose first book, "The Ink Truck," was edited by Mr. Doctorow in the late 1960s. The two have remained friends since.

"I just find him one of the best contemporary novelists," Mr. Kennedy says. "He invariably is a good storyteller. His prose is wonderfully intelligent and extremely readable, but also always with serious weight to the work."

Like his friend, Mr. Kennedy bases his fiction in the past, and both wonder at times why they've encountered so much curiosity about it.

"Sometimes I give readings, and in the question-and-answer period I often get asked why I write about the past," Mr. Doctorow says in an interview in an apartment he keeps near New York University (he also has the house in New Rochelle and a beach place in the Hamptons). "So I've developed a couple of answers. The first answer is that we live in it. That's a very easy answer, for who has not looked in the mirror and seen the past in your face?

"The Bosnian Serbs are still fighting battles that originated hundreds of years ago. In their fighting now, they're killing Turks [their historical enemies]."

I'm not the kind of writer who can walk into a room and immediately know the brand names of the clothes people are wearing -- and then figure out who is sleeping with whom, and then go home and start writing a novel 15 minutes later. I'm very slow. It takes a long time for me to figure out what happens."

At 63, Mr. Doctorow looks very much the respected literary figure, professor (he still teaches at NYU) and former editor (of Norman Mailer, Mr. Kennedy and James Baldwin, among others). He favors casual slacks and cardigans; his silver beard is neatly trimmed. He is low-key and unassuming, with a dry wit (asked to explain a character in "The Waterworks," he answers with a smile: "I don't know. It's hard enough just to write these books without having to explain them, too").

His speech is much like his writing: considered and thoughtful, if often indirect. He'll give a succinct answer, but it may come after musings that appear at first irrelevant but then are shown to be entirely on the point.

Outside on this bright spring day, Greenwich Village is alive with human activity. The basketball and handball courts are jammed with competitors and spectators; the outdoor cafes have few empty seats. Mr. Doctorow looks out a window, then offers that )) the Village's history as a bohemian center and its bustling street life helped him evoke the feel of New York in the 1870s in "The Waterworks."

"I wrote a good deal of this book here," he says, holding his hand up to indicate he means the apartment. "As a matter of fact, it was very important for me to be here at one point -- just walking around lower Manhattan to get a feeling of the streets.

"I seem to have always written about New York," Mr. Doctorow continues with a slight shrug. "It just kind of fuels my imagination. But there are different New Yorks."

And it's a decidedly different New York that he writes about in "The Waterworks." Henry James wrote about New York during this time, and so did Edith Wharton. But their mannered, genteel society was considerably removed from that described by the newspaperman/narrator McIlvaine in "The Waterworks":

As a people we practiced excess. Excess in everything -- pleasure, gaudy display, endless toil, and death. Vagrant children slept in the alleys. Ragpicking was a profession. A conspicuously self-satisfied class of new wealth and weak intellect was all aglitter in a setting of mass misery. Out on the edges of town, along the North River or in Washington Heights or on the East River islands, behind stone walls and high hedges, were our institutions of charity, our orphanages, insane asylums, poorhouses, schools for the deaf and dumb, and mission homes for magdalens. They made a sort of Ringstrasse for our venerable civilization.

"There was a certain point toward the end of this book where I said to myself, 'This is not Edith Wharton,' " Mr. Doctorow says. "None of the people, for instance, in [Wharton's] 'The Age of Innocence' would think about what was going on in New York at the time. So everything those people were not thinking of is in this book.

"This is a counterpoint, in effect, to the Jamesian, Whartonian strategy of staying with money or at a high enough level in the money so you can watch people climbing up desperately into it. But they never climb above those matters, except to be somewhat critical of those who are not established."

He leans back in his chair in the sparsely furnished apartment. "These are strategies that reflect in a certain sense the values of the authors. And, of course, what you leave out in a novel is just as important as what you leave in. I kind of left that upper-class world out in order to stay with the people who had kind of dropped out, or with the people who had aspired to it. And also the newsboys and the Civil War veterans."

Though it seems that Mr. Doctorow writes only about New York, his first novel was a Western -- "Welcome to Hard Times," published in 1960. He had spent three years as a reader in the story department of Columbia Pictures, going through hundreds novels and screenplays. Out of that experience came the belief that he could do as well, if not better, than the people he was reading.

He spent several years as an editor with the New American Library and the now-defunct Dial Press in the 1960s before quitting to write full-time. From his editing experience, he says, he learned to take apart a work and put it back together. He also learned how to respect an author's voice, which may account for his unusual ability to write novels with distinctly different tones.

Though he was a young editor at the time, Mr. Doctorow says he worked easily with Mr. Mailer, Baldwin and other authors far better known than he. Asked for his memories of working with Mr. Mailer, Mr. Doctorow nods and smiles.

"Mailer, I joined at the end of his writing 'The American Dream,' " he says. "I read that book in galleys, and I said, 'You could really make this better.'

"And he said, 'How?'

"He had revealed the guilt of the protagonist right at the beginning of the book. And I said, 'That should be withheld until the end.'

"And he agreed. But he told me I was too late, that he was tired of working on the book. He said, 'Where were you six months ago?' "

While editing, Mr. Doctorow also learned how to trust one's instincts, and how writers need to determine their own way of operating. Perhaps that's why he comes across as such a self-confident writer -- not unattractively so, but possessing a serenity that comes from decades of studying his craft.

For instance, because of the attention to detail in his books, it's often assumed that Mr. Doctorow does extensive research.

Not at all, he says. What he does like to use in composing his fiction are photographs.

"The fact of the matter is that every one of the periods that I've written about have been awfully well photographed," he says. "I know some artists, and people are always surprised to hear this, but artists take a lot of photographs. And they pin them up next to their easel, and they begin to paint off them. Of course, they're not really painting the photographs -- they're doing something else entirely different.

"That's basically what I do. For this book, I found some wonderful photographs of lower Broadway. They helped me picture the traffic, the crowds."

The rest was up to him. "The surprising thing is how little you

need to create an illusion," Mr. Doctorow says with the matter-of-factness of one who began a great work by contemplating the house he was living in. His book was going nowhere, so, as he explained in a Paris Review interview, he began to write about the wall he was facing: "One thing led to another and that's the way that book began, through desperation to those few images."

Now he's written a novel that came out of his considering a reservoir, of all things. "I wrote the book to find what I would write about," Mr. Doctorow says. "You always work best when you don't quite know what you're up to."