Your Money Or Your Life
To boil down some of the principles behind their program:
Enough is better than a feast... because it doesn't give you indigestion. They graph a 'fulfillment curve' to show the more stuff makes you happier when you have nothing - because being cold, hungry and naked really sucks. But when you already have too much, getting more stuff makes you unhappy. (For The Bargain Queen, 'too much' was when her study was so full of miscellaneous useless crap she had no space and worked on the dining table instead!) They argue convincingly that knowing how much is enough (basic needs plus the luxuries that really matter to you) is the key to feeling happy about your stuff.
You only get one life... and the more of it you trade for money to spend on random crap you don't really need, the less time you have to do the things that really matter to you, whatever they are. They show that trading your life for money can be a dissatisfying way to live, because once you have enough, buying more won't make you any happier.
Spending less allows you to save more... by eliminating unnecessary and unfulfilling spending, ordinary people can save enough money to retire early and do what they'd really like.
The Bargain Queen's quick summary doesn't really do the book justice, so she strongly recommends reading it for yourself. You can get it on Amazone by following the links, or better yet, borrow it from your local library!