He gets a little into the weeds at the end of it, but you would probably enjoy the first couple chapters of Spinoza’s ethics. In the book he’s trying to create a solid quasimarhematical foundation for ethics based in his understanding of how the human mind relates to its outside world. His conclusion is that the world of the mind and the world of external reality run parallel to each other. He ‘solves’ Descartes’s mind-body problem by saying there is no problem, they are just two totally separate ways of talking about the same thing.