I realize that the excerpt is from a book, but still you posting it suggests you agree with the whole excerpt, so:
"The convention which framed the Constitution of the United States was composed of fifty-five members. A majority were lawyers-not one farmer, mechanic or laborer."
You imply that more average people should have been involved in the framing of the Constitution? Pretty thinking like farmers, mechanics, and laborers should have had a say in the framing of the Constitution is, might I suggest, a rather simplistic view to hold. Say you were responsible for building a nation, would you give the average Joe on the street a say in matters of which they have no understanding? In economics, defense, health care, foreign policy, government, city planning, rights of people, etc? Think about today, does the average Joe even care to deeply think about many of these issues?
"Forty owned Revolutionary Scrip. Fourteen were land speculators. Twenty-four were money-lenders. Eleven were merchants. Fifteen
Is every farmer, mechanic, laborer going to be some sort of Saint, wishing to better the whole of society? Not being rich and powerful is not a formula for proper moral and ethical thinking. As far as slave-holding goes, please consider that they had no better example to grow from; that was the culture of their time. Slavery can never be condoned in any form, but its existence can at least be accounted for by historical and cultural context. Please also note, that this isn't a defense of any deplorable acts that may be clearly spoken against in the Quran; rather this is to point out the simplistic thought process behind the excerpt you posted.
"They made a Constitution to protect the rights of property and not the rights of man."
Yes, since the US Constitution is the Constitution for a capitalist state, not an Islamic one. Again, the consideration of appropriate historical and cultural context is, I suggest, important here.