I recommend Heart of Dominance and especially The Dominance Playbook for actual D/s relationship stuff.

Now that I have a partner who is into this stuff too I'm doing a lot more research on how to actually train a submissive. I've been doing bdsm with friends and partners for years, but this is my first time doing actual power exchange in a relationship. I'm confident that I can handle it, but I've been doing a lot of research. You can never do too much research.

These two books really really stood out. All the other books I've read on the topic of submissive training have felt like shitty roleplay. Other books I've found useful have been really good for teaching an individual skill, like beginner rope, or hypnosis, or how to hit someone, etc. But they don't cover the actual important skills of confidence, communication, psychology, relationship skills, etc. This is the only book I've found that discusses those things, and makes them practical, applicable, and immediately useful for controlling people.

The books (especially The Dominance Playbook) just hand you all the tools you need to actually do it. Actually submit and train someone, in a way that works, for the long term mutual enjoyment of both parties. It doesn't feel like this sort of advice, handed out on a silver platter, should be legal. Yet there is no specific law prohibiting it, and there it is, available on Amazon or pirateable off Anna's Archive.

So I actually recommend that the gooners read up. It's pretty neat. If nothing else it might improve your worldbuilding.