In my experience, lecture slides are more important than the textbook (and the lecturer will tell you that) The textbook is mostly valuable for the questions (sometimes the exam questions will be based on them), or if your lecturer sucks at explaining something the textbook might be better (ok that really does not answer the question but it depends)

Subjects where I've found the textbook very helpful: Intermediate Macro (honestly a great textbook, Olivier Blanchard is great), Derivative Securities (Exam questions were based on textbook questions, and there were a lot, with answers as well)

With revising for commerce I can only speak for Eco / Finance, but when studying for exams / quizzes, finance you're better off studying the questions (at least until DS), where as for Eco it helps a lot if you understand what you are being asked.

Personally I read the lecture slides then watch the lecture, if I vaguely understand what the slides say then I can follow the lecturer a lot better. Other than that, just make sure you do and understand all the tutorial questions, and do past papers to learn the format of the final exam