Anybody who has done ST6 will easilly feel that the ST9 textbooks are quite descriptive for some concepts and not rigorous enough. It may be helpful to look up some staff in the Hull ST6 texbook e.g 1) PCA 2) Credit Metrics etc That only applies if you believe in rigour like myself, I find that Hull is more rigorous and therefore deepens my understanding. Surely ST9 is not supposed to be focused on the Math, but I bet you will find it easier to answer a question on PCA if you have worked through it yourself! Hull also includes exercises to test your understanding.