Murder by Gaslight

Plagued by rumors that she poisoned her fiancé, Lucinda Bromley lives on the fringes of polite Victorian London society, tending her beloved plants--and occasionally consulting on a murder investigation. For the notorious botanist can detect almost any type of poison, especially those derived from plants. But the murder of a lord has shaken Lucinda: at the scene, she picks up traces of a toxin from a very rare species. So rare, in fact, that only one specimen exists in England--and it was stolen from her conservatory last month. To keep her name out of the inquest and to find the murderer, Lucinda hires fellow Arcane Society member Caleb Jones, who runs a psychical investigation agency. But as desire blooms between Caleb and Lucinda, they are drawn into the dark heart of a deadly conspiracy that can be traced to the early days of the Arcane Society. (From the publisher's description)

Young Arthur Conan Doyle battles the wicked forces of the Dark Brotherhood to defend the Crown. He is aided by Queen Victoria's best agent, Jack Sparks, a man of keen wit who bears a great resemblance to Doyle's later creation, Sherlock Holmes. Demons, zombies, and the Dweller on the Threshold make this a more pungently purple mystery than many in Victoriana.
From the co-creator of the television series, Twin Peaks. The story is continued in The Six Messiahs.

He's sardonically brilliant, more than a little handsome and possesses absolutely no memory of his former life as a London police detective. He must enlist the aid of the formidable Hester Latterly, lately returned from nursing in the Crimean War, to help him solve the murder of a war hero.
First book of the William Monk series.

This volume introduces Mary Russell, a wealthy orphan, and delightfully explores what happens when she is befriended by her retired neighbor, Sherlock Holmes. The first in a series.

To Sherlock Holmes, she was always "the woman," and truly she was the only woman he ever believed his equal, having smartly outwitted him in "A Scandal in Bohemia." Accompanied by her faithful chronicler, Penelope Huxleigh, this bold American actress mends a broken heart after solving the most regal of crimes.
First of the Irene Adler series.

It was the time of the Great Game of European powers. Spies and soldiers traded information, risking betrayal and death. Into this unsettled territory trips Amelia Peabody, intrepid heiress and would-be Egyptologist.
First of the Amelia Peabody series.
Also available as an audiobook.

"You have been in Afghanistan, I perceive." With these few words, Holmes astonishes Watson for the first, if not the last, time on the occasion of their meeting. A mysterious corpse without wounds and a bloody design on the wall nearby inaugurates their first case of detection.
This is the beginning of Sherlock Holmes' many adventures. It is also available as a graphic novel, video, audio, or as part of most Sherlock Holmes collections.