In today's
Washington Post, Dennis Drabelle
quotes from John Sutherland's
Curiosities of Literature, stating that "the first female detective in literature was probably Amelia Butterworth, who made her debut in Anna Katharine Green's
That Affair Next Door (1897)."

The assertion is inaccurate. Miss G. (aka Miss Gladden) in
The Female Detective (1864) by Andrew J. Forrester Jr. is commonly identified as the first female detective in fiction. Also predating Butterworth are Mrs. Paschal in the anonymous
The Experiences of a Lady Detective (1864, following Forrester by six months; attrib. to William Stephens Hayward, but there is controversy about this attribution), Catherine Louisa Pirkis's Loveday Brooke (1893, stories collected in
The Experiences of Loveday Brooke, Lady Detective), and Emma Van Deventer's Madeline Payne (
Madeline Payne, Detective's Daughter, 1884; and
Moina, 1891). One could also argue that Wilkie Collins's Marian Halcombe acts as a detective in
The Woman in White (1860), as well as Mary Elizabeth Braddon's Eleanor Vane in
Eleanor's Victory (1863).
Update. The Female Detective is available in an audiobook version from Blackstone.