The Lymond Chronicles by
Dorothy Dunnett. Absolutely amazing combination of history (the Renaissance), geography (Europe, Middle East, Russia), dashing but flawed central character, and a huge mystery that lasts all the way from Book 1 to the final pages of Book 6. Definitely the books of my lifetime. Start with
The Game of Kings and keep at it; even though the first 100 pages can be a bit hard going the rewards are incomparable.