The Iron Widow by Xiran Jay Zhao.
Think Neon Genesis Evangelion X Female Rage X Chinese History. It's by no means a perfect book, but it's entertaining, the premise is very interesting and the character work is quite good!
Chinese "history" in this case being pretty questionable, but Wu Zetian is always going to be a natural target of more or less accurate revisionism, and if there's one thing we know about her for sure it's that she was already the target of patriarchal revisioning.