The CAPM model and APT imho is what portfolio management theory is based on: valuing equities or other instruments relative to each other. Useful for pairs trading, alpha, beta, and really all risk management. His portfolio theory textbooks are good too I think.

Most serious trading strategies can be summarized by highlighting a sentence in Hull’s book. That’s how it was in the 00s anyway. It’s all just innovation was execution and not just speed. It relationships and little bits of edge on top of existing old strategies.

Stephen Ross actually seemed to provide the first real theory for risk management though. I wouldn’t be surprised if most prop trading firms are still based on that.