The premise of Steelheart is that our world has been changed forever by an event that everyone calls Calamity. There aren't many specifics about the event; there was a big burst of light in the sky, and then afterwards, some of the population developed superpowers. But with the acquisition of great gifts came a lust for power and domination, and the Epics, as they're called, turned into monsters, wreaking havoc on the portion of humanity that stayed human. They fight amongst themselves for ascendency, destroying infrastructure and driving people underground or enslaving them for their purposes. Many parts of the world have been turned to wasteland, and even the cities that have been preserved by those Epics who wish to rule a cooperative populace are no Eden. People are increasingly unable to remember the good old days, and only hope for such basic amenities as power, clean water, and somewhere dry to sleep.
The protagonist of the book is David, who is now 18 years old, but had his major run-in with an Epic 10 years ago, when he was eight. At that time (soon after Calamity), some humans, including David's father, still believed that some of the Epics would turn out to be good, would become superheroes to defend humanity from the others. But David's father's faith was shown to be misplaced, and David has spent the 10 years since his father's death at the hand of the Epic Steelheart plotting his revenge.
A big part of his plan is to find a group of humans called Reckoners, an underground organization that studies the Epics, finds their weaknesses, and assassinates them. He has tracked the activities of the Reckoners with almost as much attention as he has given to the quirks and skills of the various Epics, and now he is in position to try to make contact and insinuate himself into this band of resistance fighters. And he has a secret that he thinks will gain him a welcome: He is perhaps the only human who has ever seen Steelheart bleed.
There are two sequels to this book: Firefight, and Calamity. Immerse yourself in the world of Epics and Reckoners!
By the way, if this kind of fiction makes you happy, there are a few other titles you will definitely want to try: Carrie Vaughn's books After the Golden Age and Dreams of the Golden Age; and V. E. Schwab's book Vicious.