Which time periods are you interested in?
Mary Renault's good for ancient Greece.
Gore Vidal's got some fun ones - Burr is the antidote for Hamilton. Julian is another solid one, on Julian the Apostate. You'll wow everyone with your intimate knowledge of the Arian controversy.
Abdulrazak Gurnah, for 19th and 20th century East Africa.
J.G. Farrell has a trilogy examining the sins of the British Empire - The Siege of Krishnapur and Troubles are both excellent; I haven't read The Singapore Grip yet. Along those lines - Thomas Flanagan's' The Year of the French.
Thomas Pynchon's Mason & Dixon and John Barth's The Sotweed Factor - I think the former is underrated and the latter overrated, but they're both very funny.
Hilary Mantel's Cromwell novels, and A Place of Greater Safety for the French Revolution
Paul Kingsnorth's The Wake
Pramoedya Ananta Toer's The Buru Quartet is amazing.
War and Peace counts, too.
