For me it has to be the Shockingly Simple Math Behind Early Retirement.http://www.mrmoneymustache.com/2012/01/13/the-shockingly-simple-math-behind-early-retirement/
It summarises the entire concept.
It turns out that when it boils right down to it, your time to reach retirement depends on only one factor:
Your savings rate, as a percentage of your take-home pay
If you want to break it down just a bit further, your savings rate is determined entirely by these two things:
How much you take home each year
How much you can live on
While the numbers themselves are quite intuitive and easy to figure out, the relationship between these two numbers is a bit surprising.
If you are spending 100% (or more) of your income, you will never be prepared to retire, unless someone else is doing the saving for you (wealthy parents, social security, pension fund, etc.). So your work career will be Infinite.
If you are spending 0% of your income (you live for free somehow), and can maintain this after retirement, you can retire right now. So your working career can be Zero.
I know it doesn't tell you anything about investing and index funds or exactly what to cut back on etc, but it does highlight the very simple equation that you're trying to alter to affect one thing - your savings rate, as a percentage of your take-home pay. That is all that matters.
If you can get that savings rate to a minimum of 50% i.e. your living on just half of what you earn, then you're so far ahead of most people in society these days that it's almost unreal.
If you can push that percentage to 60% or even 70% of what you take home, then you're laughing.
The power of this was made apparent when I started to help a non-mustachian friend of mine with his finances. He was spending 78% of his monthly take-home pay and the other 22% really wasn't doing very much either. This guy has a great job, is very "straight and narrow" not extravagant or crazy but he and his wife really weren't self aware when it came to finances.
This post is the reason why I only read this forum intermittently and hardly look at the blog at all any more.