Great question. Here are the ones that come to my head in no particular order.

On Being a Theologian of the Cross by Gerhard Forde. Highly, highly recommend this one. It’s a commentary on Luther’s Heidelberg Disputation that shows the difference between a theologian of the cross and a theologian of glory - in our words, religion vs. the gospel. Powerful and devastating.

The Courage to be Protestant by David Wells. His first four books coalesced into an incredibly helpful critique of church and culture. Wells is a sociological theologian.
The Bondage of the Will by Martin Luther. Most people know Luther for the Five Solas, the Protestant view of salvation. What, unfortunately, most people don’t know is how that is applied. The Bondage of the Will is what he called “the hinge upon which everything turns”. He also said that if all of his writings were burned but one survived, he’d wish it was The Bondage of the Will. J.I. Packer’s intro is worth the price of the book alone.
Sex, Drugs, & Cocoa Puffs by Chuck Klosterman. Klosterman was a writer for SPIN, Esquire, and, I believe now, ESPN. Has some entertaining and sharp insights into culture.
A Heartbreaking Work of Staggering Genius by Dave Eggers. I read this in my early 20s and had never seen someone write with such power. The way he uses words and the way he can paint scenes is really unique. It is a fictionalized autobiography about how both of his parents died when he was 21 and he had to raise his 11 year old brother. This book was a Pulitzer Prize finalist. Eggers went on to write other highly acclaimed books as well as the script for Where The Wild Things Are. Lastly, he art directed Thrice’s album Vheissu.
Bonus:
Don’t Waste Your Life by John Piper. I think anyone in the church in their 20s should read this.
What about you?