I know the official CSS specs can be hard going when you drill down into the deep detail, but they can be useful in terms of top level detail too - there's
a table in the CSS2 spec that lists the different CSS selector types and syntax, which is actually quite useful in getting an overview. It's then a matter of choice whether to drill down into the more complex, detailed explanations of what each type of selector means and how it functions.
As far as the subset of CSS etc that is relevant to formatting ebooks, the most commonly referenced source of reliable info on formatting EPUBs that I've seen is
Elizabeth Castro's book "EPUB - Straight to the Point". I haven't read it yet myself, but I've seen others here recommend it highly. Might be worth a look, if it isn't already something you've looked at.