Start. Here. First.
If you're unfamiliar with Hero or haven't run the Hero system in several editions (like me), this book is a great boon.
I originally purchased the 2 tome set of Hero 6th Ed. After a few hours, I became overwhelmed by the page count. Then I purchased the Basic Rulebook. Once I finished read it, I had an epiphany -- once you've read the Basic Rulebook, the main system books become resource material.
BR provides a scaled-down version of the main rules, but all the basics are there: the powers and modifiers, the combat system, the initiative system, chargen, etc. BR gives you the basics of the system in very concise language.
Once you've had a session or two with the BR, and you realize how awesome Hero is for creating a cinematic game.
It really should be titled, "Buy This Book First," because it is an essential product for newcomers to the Hero System.