"Eon" may pose more of a challenge when it comes to selecting a rating than any other book I've ever read, since it consists of one half of a solid, well-written SF epic, and one half of a piece of incoherent junk. The story starts out like this: a gigantic asteroid arrives from outside the solar system and moves into an orbit around earth. The United States sends teams of scientists to explore it, and they soon find that the asteroid was a gigantic spaceship of sorts that appears to have come from our own future. Investigations into a library found on board soon reveal that the world is moving towards a massive nuclear showdown. This is the good portion of the book. It is written with intelligence, clarity, and an almost nostalgia-inducing dose of Cold War paranoia. The cast of characters is what most people have come to expect from hard science fiction: not extremely deep or dynamic, but believable nonetheless.
However, it all breaks down about halfway through the book. The story makes a wide turn involving alien invasion, parallel universes, alternate geometries, and some other stuff. The problem, simply put, is that this part of the book is too confusing. The explanations are cryptic and difficult to follow, and keeping track of all the new concepts that get introduced becomes quite a chore. Also, the characterizations collapse during the second half of the book. All of the major characters seem too ready to forget and ignore their previous lives and to accept all of the weird stuff that happens to them. One might, of course, make the argument that some enigmatic writing is acceptable and that "Eon" is a novel one that requires multiple readings, somewhat like William Gibson's "Neuromancer". The problem is that Bear doesn't have the literary style to pull such a stunt off, and I really have no desire to pour through this book time after time trying to fit the puzzle together. While I have great respect for some of Bear's other works, this one could have used some more planning and rewriting.