I've wanted to know more about the real lives of the Brontës ever since I read Becoming Jane Eyre a few years ago. I wanted something less sensationalized, more truthful, and that's exactly what The Brontë Sisters gave me. I found this in the junior non-fiction section at my library, and I think it would be suitable for kids 12 and up, but not for younger readers. It does discuss things like alcohol abuse and opium addiction, and it touches (non-graphically) on some more mature subjects that the sisters discussed in their books which shocked some readers of their day.
If you want to know more about these three remarkable authors, this is definitely a good place to start.
If This was a Movie, I Would Rate It: a high PG. No bad language, but the subject matter is not always appropriate for (or interesting to) children.