It's always embarrassing to admit that you're a part of a cult, but I'm a huge fan of MMM and his spiritual godfather Jacob (earlyretirementextreme.com) and I'm currently on track to retire in 4 years in my early forties.
Few points to remember as you browse the site:
1. Retiring early means that your post-retirement life could very well be 50 years long. Over that timeframe, 5-7% return on investments is a reasonable assumption.
2. Living on your investment income also means drawing down your principal eventually. The "goal" is to draw the last dollar from your retirement account on the day that you die (nobody actually believes that, what with passing money to your heirs, your presumably unknown date of death, and at least a small trickle of social security continuing, but that's the theory.)
3. Some of the lifestyle changes seem drastic, but you have to remember one thing: you are no longer working. Having complete freedom to do what you want each day is the whole point of retiring early.