Grammar

[Ouch!]

See my quotes above from The Elements of Style; you do not require a more definitive handbook for usage rules.

Edit: You would do well not to use Rowling as a guide for grammar rules. She's not exactly the best writer to ever touch a pen to a napkin.

Edit 2: After noticing that LunarWeaver has noticed my previous post, I have only one thing to say: the beauty of grammar is that there are multiple ways to punctuate the same sentence properly. Each of these various choices, however, will deliver a message differently. As mentioned earlier, the dash would imply an abrupt break. The reader would be queued to read the sentence and then break into the second clause--like so. A semicolon is the most formal of the options; it establishes a definitive break. The full stop does not attempt to visually connect the two clauses. It functions as the most blatant of breaks. The colon is somewhat less casual than the semicolon in this instance: it does not suggest a significant pause, and most closely ties the two clauses together. See what I mean?