I was going to say - I believe historians regard "The Making of the Atomic Bomb" as the definitive work on the subject, and Szilárd appears in the first sentence of that book. (I've only read the first chapter or two, been meaning to get around to this someday...)

Like this article, the book opens with a description of Szilárd thinking about a nuclear chain reaction while walking about London.

One of the very best non-fiction books that I have ever read.