Here's a summary of what they said. Note not only did Giuliani throw in the bible anyway and otherwise not follow instructions by naming several, his answer is the same as Clinton's.
And consider her answer. If the constitution is so important to understand, maybe a 10 year constitutional law professor would be the best candidate. See this post (reason number 10).
I think both Clinton's and Giuliani's answers are cop outs.
John McCain - Adam Smith "Wealth of Nations"
Barack Obama - Doris Kearns Goodwin's "Team of Rivals"
Mitt Romney - David McCullough's "John Adams"
Mike Huckabee - Francis Schafer's "Whatever Happened to the Human Race"
John Edwards - I.F. Stones's "The Trial of Socrates"
Hillary Clinton - US Constitution, "Federalist Papers"
Rudy Giuliani - Bible, "Federalist Papers," US Constitution and Declaration of Independence
You can watch here: