Oh don't get me wrong, if you're in uni, you're there to learn. This is it, this is when you're meant to learn this stuff, so you shouldn't be learning things just to pass or following the mantra of "P's get degrees". It reminds me of a quote, "If not now, then when?". This is really the time this stuff needs to be done and your here to become an expert in something, so obviously wider reading is a very good idea, especially if you're taking more than 1 unit in that area.
They really should only assess you on what they've taught you though. Theres also some clause in the legislation that makes it illegal for universities to require any extra charges of you at all (eg. buy your own lab equipment, excursion fees, things like that) the union was trying to tell me textbooks fall under this and thats why they cant assess you on it but i dont totally buy that. Regardless, they really should only assess you on what they covered.
I know for units like chemistry, its probably just easier to totally learn off the notes they give you because you end up knowing precisely what you need to know rather than getting muddled with the textbook. It varies from unit to unit though.