congratulations! semifinalist in goodreads' best science fiction category 2016! 
 what is this book....??? 
 i suppose it's best described as a sci fi/speculative fiction love story with philosophy sauce all over it. 
 it's also another one of those reviewing-on-eggshells books, because of all the spoilers that could possibly be spoilt, where we're going to have to dance our way around a bunch of important nouns and verbs. 
 so, this little puzzle book of genre-blending mastery... i know i wrote "love story" up there and that might make you want to give this a pass, but don't let it dissuade you! it's absolutely a love story - if this book were a turducken, the love story would be the chickeny center the rest of the meat is swaddled around, but it's a real love story as opposed to a romance - a love story in which the participants are mature and equally-matched and their love has deepened and endured over the years from that first burst of sexxytime infatuation into the calm steadiness of sweatpants and chores and family love. 
 and now it is threatened by an unforseeable obstacle. 
 but don't worry - it's got so much duck/turkey whizzbangery wrapped around it that you won't feel like you're reading some kissyface book. the largest, most visible part is the SF turkey outer layer, which takes the shape of a super-actiony thriller - gripping, exciting, suspenseful, and flashing so many scenes that left me jawdropped with hopelessness because of what an asshole science is 
 and i'm not saying i understand all of it because this brain is a garbage-brain, but the parts i did understand were so satisfying and creepy and oh no you didn't and amaaaaaanda! 
 it's a great thinky romp that definitely put an end to any inclination i might otherwise have had to even consider regretting choices i have made in my life or wondering 'what if...?' 
 because it is NOT worth it, man. trust me.