Most original, exciting, fresh, mindblowing? Doesn't have to be theory, non-fiction, or even leftist. I just wanna get to know more exciting books.
My own answers:
Non-fiction: Debt - The Last 5000 Years and A Paradise Built In Hell were paradigm shifts for me, in a great way.
Fiction: Gonna have to be The Trojan War Will Not Take Place. Yes it's a play from 1935 but it was more interesting for me than many more current books, and the anti-war message is unfortunately still fresh.
Do it! It's the only book that's really made me question whether 'it's easier to imagine the end of the world than the end of capitalism'. In particular the way it presents neoliberal welfare capitalism as an aberration from an outside perspective is really exciting.