The Debates in the Federal Convention of 1787 by James Madison
http://www.constitution.org/dfc/dfc_0000.htm
These are detailed day by day journals of the constitution convention as constitution was written.
It gives the reason behind each article
start your read from the first day and find out many surprising things.
"The Debates in the Federal Convention of 1787 by James Madison"
http://www.constitution.org/dfc/dfc_0000.htm