Although written coming on for twenty years ago, the two novels that make up this omnibus edition, "Hyperion" and "The Fall of Hyperion" are absolutely "must-reads" for all serious lovers of literary SF.
The first book sets out the stories of a disparate group of travellers, brought together to visit the machine entity/deity the Shrike at its "lair" in the Time Tombs on the planet of Hyperion. Ostensibly, this is an attempt to avert an invasion of the settled universe by a swarm/fleet of Ousters (humans who have opted out of the mainstream human culture, which is run and regulated by AIs). However, each has a personal reason to visit the Shrike (a normally fatal enterprise) and on the course of the journey, each tells their tale. Thus, the book is a sort of mini-Decameron for the SF crowd, with the author adopting a different tone for each segment. It is supremely written, each segment explaining more of the overall milieau and pushing the plot forward as well as delineating the characters.
The second book focusses less overtly on the characters of the original book, as the action broadens out into the political background of the setting and the action taking place on other worlds, as the Ouster invasion and its ramifactions develop. This is more conventional in its structure, but nevertheless riveting, and building to a highly satisfactory conclusion (though it is one of those books which you don't really want to end, so immersive is the story).
The writing is superb all the way through, brimming with ideas and packing a great deal of "sense of wonder", but also maintaining a high degree of action-packing and also, in parts, very moving emotionally (the last is not often a feature of even the best SF). And while fairly highbrow in places (the poet Keats is quite big in the storyline) it carries along the reader (like myself) who is not expert in these things without being annoying or patronising.
Perhaps the masterstroke of the books is the "character" of the Shrike, a sort of emblem of the mystery at the centre of the books (rather like the black monoliths in 2001). But the Shrike is also horrific and unpredictable, and every encounter with it is memorable.
As stated above, the books are about 20 years old. But they seem hardly dated, and the quality of the writing is some of the highest (maybe the highest) in the genre. Having read a lot of SF, and modern SF too, these really stand out for me as "core texts". I was initially put off reading Dan Simmons because I was only aware of his horror writing - don't be, this is pure, fabulous SF of the highest order.
Everyone raves about these books - there is a reason.