I've been struggling to think of any way to describe this book and the best I can come up with is that it's something like if the snarky heroine of "Murderbot" found herself a swordfighting human Gene Wolfe's "Shadow of the Torturer" trilogy and then got REALLY involved a necromancy murder mystery (with a little bit of unrequited lesbian crushing on the side).

Not sure if that's helpful.

Anyway, in Tamsyn Muir's very original setting, there is an ancient decaying empire supported by nine Houses, each of which has a helpfully numeric surname and is often just referred to formally as their number (e.g. "Ninth"). Each of the nine houses specialize in a particular type of necromancy - and I'm super impressed at the author for even thinking up the details of nine different types of necromancy. The Emperor - who has been fighting some war, the details of which remain fairly vague - used to have a team of Lyctors supporting him but over the centuries many have been lost. He needs more. A representative team from each of the nine houses are summoned to a crumbling temple on a distant planet where the rest of the book takes place, and where they are expected to figure out how to become Lyctors themselves.

Each team consists of one necromancer and one cavalier (roughly, the magic and the muscle) and the titular Gideon is in this case the cavalier to Harrow, heir to house Nine and kind of a goth asshole. (Apparently ALL of house nine are goth assholes though, so par for the course.) Gideon and Harrow are the only two children surviving their generation; two hundred of their age group died young of a plague of some sort, and House Nine is on the brink of extinction, so Harrow accepts the challenge to become a Lyctor in hopes of saving her House and Gideon is the only option to go with.

As they arrive at the testing site and compete to find and complete the tests themselves, it turns into something of a cross between the Hunger Games and a murder mystery as mysterious deaths start occurring around them. There are many questions unanswered and it takes a while to even start to figure out quite what is going on, as the world Muir has created is so strange and unusual.

This is apparently the first in what is to be a trilogy, the only one published so far at the time I write this, but although the ending leaves it clear that there is more to come, it's not a horrible cliffhanger and does bring this section of the story to a satisfactory (more or less) conclusion.

I don't imagine this book will be everyone's cup of tea, but if the above description sounds interesting to you, you'll probably enjoy it!