There are. I'm not a great mathematician, even though I'm an architect. It took me a long time to realize that mathematics is one of those fields of endeavor where you have to have a sense of humor. When you think about a concept like negative numbers -- you can't really take that seriously. Probably most of the math I know is in that tiny book, though, so don't push me on it.

With "The Dot and the Line" you did the art yourself, but with "The Phantom Tollbooth" you had someone else do the illustrations, Jules Feiffer.

That happened completely naturally. At that time Jules Feiffer and I were sharing a duplex in Brooklyn Heights. He was on the floor below me and just getting started in his career, which of course has blossomed enormously. He'd hear me pacing back and forth, because I pace while I'm writing. He got interested and I gave him a few sections. He started doing illustrations and they were wonderful. I never thought of the book in any other way. I brought the book to the publisher with the illustrations. I since learned that you just don't do that.

People always ask me if I'm going to do a sequel, and probably I won't. The sequel most people want is another trip with Milo and basically the same characters.

They'd want you to bring back Tock, the watchdog, and the Humbug. You can't blame them, they're such appealing characters.

Tock was inspired by the old radio program "Jack Armstrong." I was looking for companions for Milo to travel with and the first one needed to be stalwart and dependable and truthful and you could bet your life on him and he'd always be there in an emergency. Jack had an uncle named Uncle Jim who was all those things. He was the model for Tock. At the same time, I suddenly realized that I needed two companions. Tock is just one side of the equation. Tock was the kind of person who was the friend your mother wanted you to play with. But of course you were always attracted to the kind of kid your folks didn't want you to play with, who was not terribly truthful, always got into trouble, was not completely dependable, erratic, a fraud, a bluffer, you never knew what he was going to do. And of course, that was the Humbug.

Yet you're not up for a sequel that continues with those characters, like the Oz books.

I don't think I can do that. It's a fine thing to do. It's what J.K. Rowling is doing with Harry Potter, and those books are quite wonderful. Her books are just a shade off of reality. There aren't spectacular, intergalactic things happening, it's just a shade of what you'd expect to happen.

You keep up with children's literature?

I can't say I'm a great scholar of it, but I try to keep up with things that are interesting.

Do you want to plug anyone else?

There was a terrific book called "The Mouse and His Child" by Russell Hoban.

Did you have any inkling when you were writing "The Phantom Tollbooth" that you would have such a big influence on so many people's imaginations?

You never know that. I had the usual first book paranoia. I was convinced that the book was distributed in the dead of night, in unmarked trucks, in unidentified boxes that were immediately put in the basements of bookstores where they would never been seen again. And I don't know why this happened, but then we got the lead review in the New Yorker, in their special children's books section. Emily Maxwell, Bill Maxwell's wife, did the review, and it took off from there.

I think it surprised everyone because they said, "This book will never go. It's too difficult. The ideas are too complex and too abstract. The vocabulary is beyond children." I was perfectly content to have done a book and to have it published. But it's interesting. It's become an extraordinary part of my life. I get mail constantly and try to answer them all because I know how much it means to a kid to write and to get a reply. I don't think a day or a week goes by that something doesn't happen in relation to that book. It's been a wonderful thing.

Maya Angelou reads from "The Heart of a Woman"