Amos.

The book of Amos is one of those books that continues to be SO relevant today. I'm continually blown away by this book that has dominated my recent Bible reading. Man...don't overlook the minor prophets!
So, who the heck was Amos, anyway? Circa 765 BC: the book of Amos was written in a time when the economy of Israel was doing really well - the rich were getting richer, but many were doing this by taking advantage of the poor. The class difference was yawning wider as the rich praised God for their "blessings." Amos was basically an average joe who came to warn the country that God would shatter them because of injustice.

This is an excerpt from the introduction to Amos, as found in the message translation (I know, I know, it's not a "real" translation, but whatever). This is mostly for my benefit, because I want to have it easily accessible online, but read on, it's pretty quality.
Introduction: Amos
More people are exploited and abused in the cause of religion than in any other way. Sex, money, and power all take a backseat to religion as a source of evil. Religion is the most dangerous energy source known to humankind. The movement a person (or government or religion or organization) is convinced that God is either ordering or sanctioning a cause or project, anything goes. The history, worldwide, of religion-fueled hate, killing, and oppression is staggering. The biblical prophets are in the front line of those doing something about it.

The biblical prophets continue to be the most powerful and effective voices ever heard on this earth for keeping religion honest, humble, and compassionate. Prophets sniff out injustice, especially injustice that is dressed up in religious garb. They pay little attention to what men and women say about God or do for God. They listen to God and rigorously test all human action and language against what they hear. Among these prophets, Amos towers as defender of the downtrodden poor and accuser of the powerful rich who use God's name to legitimize their sin.

Amos 2:6-8 (The Message)
Because of the great sins of Israel...I'm not putting up with them any longer.
They buy and sell upstanding people. People for them are only things -- ways of making money. They'd sell a poor man for a pair of shoes. They'd sell their own grandmother! They grind the penniless into the dirt, shove the luckless into the ditch.
Everyone and his brother sleeps with the 'sacred whore' -- a sacrilege against my Holy Name. Stuff they've extorted from the poor is piled up at the shrine of their god, while they sit around drinking wine they've conned from their victims.

Sound familiar?

Amos 5:7-15 (The Message)
Woe to you who turn justice to vinegar and stomp righteousness into the mud.
Do you realize where you are? You're in a cosmos star-flung with constellations by God, a world God wakes up each morning and puts to bed each night. God dips water from the ocean and gives the land a drink. God, God-revealed, does all this. And he can destroy it as easily as make it. He can turn this vast wonder into total waste.
People hate this kind of talk. Raw truth is never popular. But here it is, bluntly spoken: because you run roughshod over the poor and take the bread right out of their mouths, you're never going to move into the luxury homes you have built. You're never going to drink wine from the expensive vineyards you've planted.
I know precisely the extent of your violations, the enormity of your sins. Appalling! You bully right-living people, taking bribes right and left and kicking the poor when they're down. Justice is a lost cause. Evil is epidemic. Decent people throw up their hands. Protest and rebuke are useless, a waste of breath.
Seek good and not evil -- and live! You talk about God being your best friend. Well, live like it, and maybe it will happen. Hate evil and love good, then work it out in the public square. Maybe God will notice your remnant and be gracious.