Phases of Gravity is SF in that it deals with an ex-astronaut and uses some technical jargon. Otherwise, it's a contemporary novel of man searching for meaning in his life, and a reason to live it. It's about a gray man with a gray existence, and unfortunately, the novel turns out to be pretty gray as well.
I'm a fan of some of Dan Simmons' work, but this book just doesn't offer much. It's well written, but the theme of an aging ex-astronaut's midlife crisis didn't attract me. Baedeker is drifting, emotionless, and acted upon. The main external driver of the story is a younger woman attracted to him for no particularly good reason. Baedeker himself meanders here and there, never interested in much, never doing much. It's hard to see why anyone would really care much about him, and I didn't.
The story is not helped by being composed largely of flashbacks interspersed. "Baedeker suddenly recalled" could be the theme of the story. Occasionally the story jumps forward, apparently so that the time skipped can be reviewed in flashbacks later. It's awkward, and not particularly effective.
Baedeker spends a lot of his time thinking back to his walk on the moon, and the moments that went with it. He's vaguely interested in a transcendent experience, and in how to evaluate experience in general. He's also, in a subdued way, a bit of a whiner. That is, he doesn't feel his life has amount to much beyond his brief moment of glory, but since he's so passionless, I found I didn't much care.
It's possible that there were hidden levels of meaning. For example, that the name Baedeker hints at the Baedeker travel guides, given that Baedeker is trying to find his way. But I think that gives Simmons too much credit for subtlety. I think he was really just trying to tell a story about man and middle age, but ended up with too much of the latter, and not enough of the former. By page 16, I was already finding the book dull, and despite smooth writing, it never really did get better.
The story is grouped loosely around Baedeker's reunions with his two former mission-mates. In the second, he seems to wake up a bit, but only a bit. He's generally in a bit of a fog, paying more attention to memory than to reality. Simmons has clearly done his research on proper terminology, so that the men can converse with great verisimilitude, but occasionally, he's so intent on getting it right that he forgets to keep the jargon interesting. I don't really care if an access port is called a hell-hole when I also don't care what's behind it. On the other hand, he quotes "one giant step for man" without comment, when surely an astronaut with at least think about Armstrong's claim to have said "for a man". Similarly, technology that is "light-years ahead" doesn't really underline the book's SF character.
A portion of the novel relates to a book a character is writing - noting that the heart of the book will be profiles of ex-astronauts. Clearly that's part of what Simmons is attempting, but I'm afraid his character succeeds better than he does. Simmons' introduction of colorful but extraneous characters doesn't really help.
Open Road Media's uneven proof-reading continues here, with a few dozen incorrect paragraph breaks scattered throughout. The lack of scene breaks doesn't help.
All in all, a decent look at what an astronaut's post-mission life might look life. Unfortunately if you're the dispassionate type, it mostly looks dull.