The first female detectives return.

M. E. Braddon's
Eleanor's Victory (1863),
which features a female
amateur sleuth.

Under the title The First Female Detectives,  the pseudonymous Andrew Forrester Jr.'s The Female Detective (1864) and Revelations of a Lady Detective (aka Experiences of a Lady Detective, 1864, attrib. [controversially] to W. S. Hayward) have been reissued by Scholars' Facsimiles and Reprints. Eastern Illinois University's Dagni Bredesen, who wrote a Clues article about these works, introduces The First Female Detectives.

Forrester's female detective is one Miss Gladden. Regarding Revelations of a Lady Detective, Bredesen notes in her Clues article:

. . .Mrs. Paschal’s “revelations” comprise[] ten stories in which she deals with a variety of crimes and misdemeanors ranging from thefts of gold, jewels, mail, and identity to political conspiracy, murder, and fraud. In solving these mysteries, Mrs. Paschal curbs the excesses of a too-merry widow, an errant spouse, and wayward sons and brothers at the behest of either government officials or family members. Despite her conservative politics and policies, Mrs. Paschal is an exception to the rules she takes it on herself to uphold. (20)

These important works in the history of the mystery genre have not been available in their entirety since the 19th century.  (Hat tip to Judith Flanders)