Charlotte Bronte: A Fiery Heart (Harman)
Other than Mrs. Gaskell, I had not read a full-length biography of Charlotte Bronte, so I do not know how this modern scholarly biography stands up to its competitors. However, it completely satisfied me, filling in the gaps, based on primary sources of evidence, and interpreting the whole with sympathy but not outright partisanship.
I'm actually glad I came to the Bronte biographies late. I think it would have interfered with my full enjoyment of the fiction to know how deeply rooted Charlotte's stories are in the incidents and personages of her own life, and to be distracted by the game of similarities and differences from the more straightforward enjoyment of fictional characters and plots. That said, reading this biography encourages that new game, and I may find myself at some point picking up Jane Eyre or The Professor again.
I made a few notes for myself of the more amusing interesting tidbits, and of course immediately lost those notes. I do, however, remember that many of the Jane Eyre names (including "Eyre" and "Rochester") can be found in a single churchyard that Bronte visited. Much of the understanding we have about her relationship with the Belgian employer she fell hopelessly in love with (Constantin Héger) appears to derive from letters that were preserved by his wife - a bit of an enigma there. Some of those letters were actually torn to bits and carefully reassembled (there is a photograph in the images section).
Harman also expands considerably on what Mrs. Gaskell had to say about the Brontes' relationship with their publishers, and about Charlotte's interactions with other writers. She was the only one of the sisters to become in any way part of the literary "scene" of the time, chiefly because she was the only one who lived long enough to do so.
If I were inclined to go far down the rabbit-hole of Bronte studies, Harman's book, with its extensive notes and bibliography, would be an excellent starting point.