Infinity Beach is a really, really interesting book. McDevitt has successfully melded a mystery, a love story, and some very good social and sci-fi commentary in a way that is both surprising and rewarding.
The first half of the book sets up the mystery. The main character, Kimberly Brandywine, becomes increasingly involved with the 25 year old disappearance of her sister, Emily. Emily had gone on one of the few interstellar missions at that time still looking for first contact. The mission had returned early and unsuccessfully because of engine trouble. Emily ended up missing just a few days later after a still-unexplained explosion ripped the side of a mountain off, decimating the city she was visiting.
There are considerable overtones of possible first contact in this part of the story. But McDevitt lets us chew quite a while on the possibility that life on earth might be unique; a question not often considered in this genre. McDevitt does not stress this question overly much. Instead, he spends significant time inspecting the societal impact on humans in a future where, after considerable effort, no contact has been made with any type of lifeform other than from earth. Every world humans have visited have so far proved completely sterile.
In Infinity Beach, humans have given up on finding other life, at least within their lifespan. The society he describes has slowly relaxed into a world of virtual pleasures and work-free luxury.
The only negative I had with Infinity Beach was that at this point, about mid-book, the story has slowed down quite a bit; if you feel like quitting here, keep going (it will be worth it!)!
The second half of the book builds the suspense as Kim gets closer to answers. A love story and more scifi elements get involved when she follows the path of Emily's last voyage. Quite a ways from the end of the book, McDevitt starts providing some answers to the mystery and all its threads. These last hundred pages produce a number of great plot twists, and are fun (in a couple different ways) and depressing (in a couple other ways!) all at the same time.
Infinity Beach is one of those rare books that does not just "stop" on you. The conclusion to the story winds down at a very nice pace, giving plenty of time for the reader to consider the consequences of the characters actions, evaluate the state of the human race, tie-up various hanging threads in the story line, and to "detach" from the story. The slow buildup at the beginning of the book seemed finally worthwhile, as the end reciprocated in a similar and very rewarding fashion.
Infinity Beach is a great sci-fi book, with a lot of elements and pacing that are rare in the genre. Highly recommended.