This book is a quick read.
The major point of this book is to select stocks which have the lowest P/E's relative to their total return on all capital. It makes a lot of sense. As an owner of any business, you want the best returns on the money you have invested in the business. You also want to buy businesses that are cheap (thus low P/E) relative to their returns.
The way Greenblatt suggests doing this is mechanistic by changing your holdings at regular intervals given the return on capital and P/E's of the stock universe at the re-evaluation time. A key point is the relative P/E's over time. Prices of equities can fluctuate to a significant degree over time. That fluctuation can be extensive even over short periods. The result is fluctuating relative measures of value.
While the returns are really great over longer periods of time, Greenblatt clearly admits that there are times that the method will be out of favor. Sometimes these time periods can be several years, so many investors choose not to follow this method. But... Greeenblatt has found this method to provide outstanding returns over extended periods of time.
Ultimately the goal of portfolio management is to achieve a great long term return. Greenblatt does a great job showing how it can be done - simply.