The main point of Ms Leonard is that those in their 20s and 30s should start investing for their retirement now. This is important due to the power of compounding which effect is all the greater the longer the money is invested. It explains how a target of one million dollars is achievable if one starts investing early enough. And the beauty of Ms Leonard's idea is that one need not invest large sums to achieve the target (if the investments start early).
It is a great general introduction to investments, shares, bonds, unit trusts etc. And there are some lessons here even for more experienced and knowledgeable investors.
There are two downsides, which is why I have taken a star off for each. First Ms Leonard does not emphasise directly the dangers of active management (using actively managed instead of passively managed/indexed funds means that fund manager's fees eat into an investors funds considerably - see 'The Succesful Investor' by Larry Swedroe if you are in any doubts about this) and the book should emphasise this in my view. The second is that the book relies on a crucial assumption to prove that you can reach the target of one million dollars - that investments will continue to return 10-12%. This assumption has been shown to be incorrect even since the book was written roughly 12-ish years ago. Nevertheless what Ms Leonard advocates in this book certainly has much to recommend it, and the principles hold true even if this assumption is wrong!