Dark Quartet is a fictionalised biography of the Bronte family.
The historical details of the lives lived at Haworth Parsonage are well documented in various biographies, and no doubt all of them have some degree of imaginative embellishment.
This book takes those facts previously documented and interprets them with a novelist's skill, so that the thoughts and feelings she invests each one with has an authentic ring. I read the chapters detailing the deaths of Branwell, Emily and Anne with tears running down my face
However, even Banks doesn't come up with any theory as to what made them all quite as odd (or worse) as they were!

This book stops where Charlotte returns to the Parsonage after Anne's death, and the story of her years of fame, are continued in another book: Path to the Silent Country - which I will definitely find and read on some future occasion