I don’t think there ever was such an apple of discord as that unlucky book. (Elizabeth Gaskell to George Smith, 26 November 1857)

I am curious 

about the backgrounds

of the biography

Elisabeth Gaskell 

made from Charlotte Bronte

I am going to search for background information.

On 16 July 1855, Patrick Brontë wrote to Mrs. Gaskell, acknowledging her as the person “best qualified” to write an account of Charlotte’s life and works, and asking if she would agree to the task (SHB. XV. 190-191).  Within a week, Gaskell arrived at Haworth Parsonage to discuss the biography.  From the beginning, she intended to do much more than  the “brief account of [Charlotte’s] life and . . . some remarks on her works” for which Patrick had asked

(XV. 190).  She wrote to Ellen Nussey:

I told Mr Brontë how much I felt the difficulty of the task I had undertaken, yet how much I wished to do it well, and make his daughter’s most unusual character (as taken separately from her genius,) known to those who from their deep interest and admiration of her writings would naturally, if her life was to be written, expect to be informed as to the circumstances which made her what she was (Letters 361)

Gaskell’s biography was to be a tribute to both the woman and the writer; she also intended it to be an expression of their treasured friendship, as she said in her letters both before and after the book’s publication.  Indeed, before she had even been approached by Patrick Brontë, she had written to Charlotte’s publisher, George Smith, that she longed to “publish what I know of her, and make the world . . . honour the woman as much as they have admired the writer” (345).  And when the “unlucky book” was published, and Gaskell was “in the Hornet’s nest with a vengeance” (453) from those unhappy with their particular portrayals, she stood true to herfriend’s memory, asking Charles Kingsley to “[r]espect & value the memory of Charlotte Brontë as she deserves” (452), and telling Ellen Nussey:

I weighed every line with all my whole power & heart, so that every line should go to it’s [sic] great purpose of making her known & valued, as one who had gone through such a terrible life with a brave & faithful heart. (454)

Gaskell had mourned Charlotte’s passing; indeed, she most likely still mourned her, or at least she expected to: My dear dear friend that I shall never see again on earth! . . . I loved her dearly, more than I think she knew.  I shall never cease to be thankful that I knew

her: or to mourn her loss. (335-336) Thus, as Pollard says, Mrs. Gaskell wrote with a “tender concern” that highlighted the “profound attachment” she had for Charlotte Brontë (Mrs Gaskell 142-143), a woman she considered “truth itself” (Letters 128).  If, indeed, her purpose was to exalt Charlotte’s goodness (Pollard 146), it was also certainly sincere.  Gaskell’s purpose was to write an account of Charlotte just short of hagiography, and she never claimed otherwise: I appeal to that larger and more solemn public, who know how to look with tender humility at faults and errors; how to admire generously extraordinary genius, and how to reverence with warm, full hearts all noble virtue. To that Public I commit the memory of Charlotte Brontë. (Life 429) Wise and Symington state that, though the friendship between Elizabeth and Charlotte was brief, “[n]ever, anywhere, do we find a single jarring note” (SHB. XV. 61).  It is appropriate that the friendship between these two extraordinary women be emphasised, for in the nineteenth century, a friendship like theirs was considered most rare. lang/EG

Here you can read an opinion completely different: guardian/classics.charlottebronte 
Since her death 150 years ago, Charlotte Brontë has been sanitised as a dull, Gothic drudge. Far from it, says Tanya Gold; the author was a filthy, frustrated, sex-obsessed genius.
Elizabeth-Cleghorn-Gaskell