The main point of this book is that you should be following Modern Portfolio Theory, which, as is mentioned a few dozen times, earned a Nobel Prize for the economists that came up with it.
This book was too long and was highly repetitive, but the two main points I took away were to invest in index funds and to focus on asset allocation for performance over trying to pick a winning fund or sector.
We are told to invest in index funds since the fees are lower and since, in the long run, actively managed funds perform worse. This is backed up by a bunch of various studies that he quotes. Of course, we know that if you picked the best activae funds while they were the best, you'd have outperformed the market, but picking a winner is close to impossible and the top performers one year can literally be with worst the following yar.
Regarding asset allocation, the book provides a couple reference portfolios and shows how, mathematically, how it's possible to lower your overall portfolio risk while increasing your possible return.
I think this book could have been half the size.