I have the book he cowrote. Wondered the same thing. He can "see" numbers like shapes which creates a final shape in his mind which he can evaluate as a number of large length.
He memorized PI to 20000 digits and said it outloud in an attempt to gain donations for autism.
The book is called "Born on a Blue Day" by Daniel Tammet.
Seems to be published in the U.K in 2006, I got mine at a book sale in Canada.
