There aren't actually any ghosts in this book. If you're looking for spooky horror/fantasy IN SPACE, this is absolutely not the book for you: all ghosts are purely metaphorical.
What this book actually offers: American-style slavery, racism, intersexism (kind of combined with racism, in this case), whatever -ism goes with mental illness and neurodivergence, transmisia, homomisia, &c. .... IN SPACE.
(I don't mean to imply that it condones these things. Quite the opposite, in fact. But it certainly does portray them.)
The hard sci-fi setting serves more as a backdrop for all this slavery and oppression and whatnot than as a foreground element, despite its plot-relevance; possibly I feel this way because the plot itself takes something of a background role as compared with social themes, relationships, and the constant crushing realities of slavery. Thematically, though, it is important. Even knowing that the vast majority of slaves never escaped, there's a difference - at least from a literary perspective - between being enslaved beneath the open sky with only a lot of distance, a whole bunch of oppressors, and an entire society built around your lack of personhood between yourself and freedom; and being enslaved IN SPACE, with recycled air and the world curving in around you, and literally no freedom anywhere because you're trapped on a spaceship and "a whole lot of distance" just straight-up doesn't exist. The setting does quite a bit to enhance the feelings of hopelessness and inevitability in a subtle way.
It's worth noting that despite its themes and subject matter, An Unkindness of Ghosts is actually surprisingly easy to read. While the violence inherent to the subject isn't glossed over, the worst of it almost entirely takes place off-screen. I wouldn't call it a pleasant book, but the unpleasantness certainly isn't gratuitous, and in fact it generally feels somewhat toned down for readability.
The prose is fluid and interesting, and all the more interesting because of the use of dialects and varied styles of speaking, with differences between various decks of the ship. Rivers Solomon obviously pays a lot of attention to language. Aster is a literal-minded pedant with elevated diction (fair enough: autism does sometimes present this way), which had me worried for the first few pages - I feared Autism Voice and dubious clichés - but as I realised that 1) she has a hell of a lot more character depth than just that and 2) she is only one of many characters with a distinct voice and her literal-minded pedantry and elevated diction don't result in her looking down on those who use figures of speech or speak in dialect, my worries soon faded.
All around an important book and one much worth reading, as long as you can put up with a bit of soul-crushing oppression in your literature.
Selling points: cool lingual stuff; sci-fi-but-also-low-budget-herbal-type medicine; intersex autistic protagonist; slave revolts; psychotic (schizophrenic?) representation with depth; there's a scene with a dog (and nothing bad ever happens to the dog!); transfeminine representation; strong focus on consent in both sexual and medical situations; not nearly as unpleasant to read as one might assume given the subject matter.
Warning points: honest depiction of slavery and all that comes with it; violence, including sexual violence (though especially the latter is mostly off-screen); there's a (consensual, and not particularly graphic) sex scene in there somewhere; the psychological violence is not just off-screen; descriptions of medical implements and procedures (and also of the causes for them: wounds, gangrene, that sort of thing); plenty of references to self-harm and suicidal ideation; semi-graphic on-screen character death; social friction; soul-crushing oppression.