Hyperion by Dan Simmons
I have to say this might be the book (among both SF and non-SF books) that had the biggest impact on me.
The story in a few words: among the hundreds of billions of humans spread over the galaxy, seven pilgrims are “randomly” selected each year to travel to the mysterious Time Tombs located on the planet Hyperion. Nobody has ever seen any of the pilgrims coming back before. Thus, our seven pilgrims decide to share their own past (a la ‘The Canterbury Tales’) to find out a common thread that could save them. Add to that the imminent attack of the Ousters (a branch of humankind that left hundreds of years before to live in the void of interstellar space), the influence of the millions of AI forming the TechnoCore, the deadly Shrike following the pilgrims in the shadows and the cybrid (clone) of the late poet John Keats and you have one of the best space-opera/humanist book ever written.
Each chapter of the tale of the pilgrimage to Hyperion is followed by the story of one of the pilgrims and each one of them is written in a different literary style (one is a noir cyber detective story, another is a personal diary, a love story, …).
The first book (which won the Hugo prize in 1989) was immediately followed by 'The Fall of Hyperion" and a few years later by another cycle taking place a few centuries later (“Endymion” and “The Rise of Endymion”).