I'm 50 years old and know that I could have benefitted greatly from reading this book when I was 30!
The book is a wealth of knowledge for those wishing to explore the option of retiring early - decades before most people are able to do so. Although relatively short (150 pages) it's packed full of practical, easy to use information. The book covers the whole gamut of retirement planning. It touches on everything necessary to put you in a position of retiring early including such things as exploring your spending habits, the importance of consciously making decisions to bring you more happiness, how to reduce your spending, the calculations needed to determine when your ready to retire, and considerations that one should make when they think they're ready to pull the pin.
There are other good books out there that supplement this book. The book that comes to mind is "Your Money or Your Life" by Joe Dominguez and Vicki Robin. Free at 45, however, addresses some things outside the scope of the Dominguez/Robin guide to financial independence and I particularly enjoyed the information on the Canadian OAS and CPP - two programs that are often not discussed in similiar books that originate in the United States.
I was happy to discover that I'm closer to being able to retire then I thought. Rather then basing your retirement needs on some percentage of your existing income, the book gets you to evaluate your spending habits to more realistically determine your needs. Although I don't plan to retire in the next few years, it is comforting to know that the option may exist.
I would recommend this book to anyone interested in retiring early. The book requires you to reflect on what you really want, what makes you happy, and the importance of making cognitive decisions when it comes time to spending. A simple read and hard to put down once you've picked it up. Very insightful!