Ancillary Justice is The Left Hand of Darkness turned upside down.
Mild spoilers for both The Left Hand of Darkness and Ancillary Justice after the readmore (both books you should read but I don't think this post would ruin them for you. Up to you and your own spoiler tolerance I guess)
- They are both (at least in part) about the POV character going to a planet which is not a part of their interstellar society to try and get them to join. In Ancillary Justice this is explicitly a very awful and violent and unjust process.
(I think there's some debate to be had over whether the Ekumen's process in TLHOD is just - they very explicitly will not take no for an answer and will use stasis and time dilation to their advantage to keep sending someone every generation until they're successful. But it's definitely much less overt and even if you do make that interpretation it's not central to the text. I don't know if this gets expanded upon in the rest of the Hainish cycle I'm a fake fan I haven't read any other books in it) - They're both about interactions between gendered and gender-neutral cultures. The Left Hand of Darkness gives a biological explanation for the gender-neutrality of Gethen, whereas Ancillary Justice makes it a purely cultural phenomenon. Additionally, Ancillary Justice makes our POV character be a part of a gender-neutral society looking out, rather than an outsider looking in.
- In The Left Hand of Darkness the default/neutral pronoun is he/him. In Ancillary Justice it's she/her.
- Both books make really good use of how they reveal information to the reader and in what order. Both require you to read what isn't written on the page as much as what is.
- Both are revealed a good ways in to the story to have been a romance the whole time.
