Please note the corrections in the quoted text above.
The Harry Potter books were written by a British author. They are among the best selling books in history and have probably been proof-read rigorously. It's very unlikely you'll find a grammatical mistake in any of them (unless intended and meant to be understood as such).
No. There is a potential difference between those two forms: "stop it raining" suggests it's already raining (or that it might rain in the future), whereas "stop it from raining" is more tentative and would suggest that the speaker is trying to prevent a future event. Since, I would imagine, the book's context clarifies what the author means, the first form is adequate.