Sanctions do work in countries where civil liberties are not completely suppressed, where the monopoly of law enforcement, military power and information is not absolute. There you can hope for civil unrest and rioting, otherwise you obtain nothing but rallying with the leader and resentment and hatred towards those who are enforcing the sanctions. There are countless examples in modern history, ranging from Italy in the 30's to Iraq in the 90's, passing through Rhodesia, Libya (in the 90's), Serbia, take your pick.
Back to the Iraq example, thirteen years of sanctions deeply affected the population as testified by FAO, UNICEF and pretty much any NGO who was allowed to visit the country. And yet, Saddam kept his place until overturned (surprise) by an invasion.
Incidentally, I've read that book, and quite a few others, during my military training. It's an amazing read, albeit utterly out of date. At any rate, I honestly didn't remember it advocates that sanctions work better than invasion, are you sure about your quote?
The wealthy class is who is usually first to turn on the sanctioned government, not the poor. The wealthy are who change things, and they will always change things to suit themselves. Maybe the poor riot, hopefully they don't starve - international humanitarian aide (e.g. dropping bags of rice) is pretty savvy these days... Plus there is always the assertion that a sanctioned government whose people are suffering is a government that requires change - the whole point of sanctions.
Sanctions make change more suitable for the decision-makers (Oligarchy) than doing nothing. Sanctions work or they would not be used. Of course the poor suffer first and most, that's always the case in peace or war, right or wrong.
I would speculate that the oligarchy (Sunnis) of Saddam's era were not 'sanctioned' enough to confront him during those 13 years.
(It was incorrect for the USA to leave Iraq during the first invasion 20 years ago at the UN's behest. The Bush family had to regain the Presidency to right that wrong. They had death threats from the Shiites that were abandoned in the South of Iraq because WE left them to Saddam. Aweful 13 years of suffering and murdering. I don't wonder why Bush Jr. lied in order to attack Saddam, its obvious to me. Back to topic...)
Know that I have not been to Iraq and cannot speak from first hand experience.
Art of War states that removing an enemies capability to wage war is plan #1, and %100 more effective than the always ill-fated occupation of enemy territory. It's just about the first thing stated in the book; (we call it passive aggression these days).
Did you go to Iraq during the invasions?