In the world of The Black Tides of Heaven, children are born without gender. They usually choose whether they want to be male or female by the time they’re in their mid- to late teens. Some children get their gender as young as three and some don’t decide until after their teens. There’s a vague reference to this involving a trip to the doctors, so this seems to be a world in which being trans is the norm. It creates an interesting effect since readers won’t know which gender, if either, a character will pick once they get older.

The magic in this world is known as slackcraft and it can be used by a woman to get pregnant by herself, to shape how trees grow, to make things float, to start fires with your mind, to kill with a thought, and much more.

Twins Mokoya and Akeha can communicate with each other telepathically. They’re very close to each other and make a vow to never choose a gender. They’re partly raised in the monastery and partly raised in their mother’s palace. Mokoya has mismatched eyes and the ability to see the future. The future can’t be changed, however. Things always turn out as they do in Mokoya’s visions no matter how much someone tries to stop it. The characters have an interesting discussion regarding whether free will exists in their world or not due to these unchangeable prophecies.

The story begins with the birth of the twins and continues throughout their childhood and into their adulthood. The twins are close to each other, but they do fight like regular siblings do. A wedge starts to form between them when Mokoya decides to pick a gender and Akeha still doesn’t want to.

I didn’t understand why they decided to kill the nearly-extinct kirin (a half-bird, half-lion creature) or the giant naga when they could have presumably used slackcraft to repel or trap these animals instead. I felt like they resorted to killing far too quickly when it comes to humans as well. Life must not be considered as valuable in their world as it is in ours.

As the story progresses, we learn of a group called The Machinists who believe everyone should have access to technology, not just slackcraft users. The Machinists start inventing technology which doesn’t require slackcraft in order to work. This reduces the power of the monastery, but the monastery supports them anyway.

The characters realize this technology could be very dangerous if it falls into the wrong hands, but unlike most fantasy or sci-fi, they’re realistic enough to realize that someone else would just end up inventing it eventually, so destroying the technology won’t really solve any problems.

I liked this book overall. Recommended.