There are 4 books in the Jackaby series. The newest book is The Dire King which came out in 2017.

1
Jackaby

Book cover of Jackaby
Alone and newly arrived in New Fiddleham. 1892, Abigail Rook finds work as the assistant to R. F. Jackaby, an investigator of the unexplained with the ability to see supernatural beings. On her first day, Abigail finds herself in the midst of a thrilling case: A serial killer is on…
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2
Beastly Bones

Book cover of Beastly Bones
"I've found very little about private detective R. F. Jackaby to be standard in the time I've known him. Working as his assistant tends to call for a somewhat flexible relationship with reality." In 1892, New Fiddleham, New England, things are never quite what they seem, especially when Abigail Rook…

3
Ghostly Echoes

Book cover of Ghostly Echoes
Abigail Rook and her eccentric employer, R. F. Jackaby, dive into the cold case, starting with a search for Jenny's fiance, who went missing the night she died. But when a new, gruesome murder closely mirrors the events of ten years prior, Abigail and Jackaby realise that Jenny's case isn't…

4
The Dire King

Book cover of The Dire King
The fate of the world is in the hands of detective of the supernatural R. F. Jackaby and his intrepid assistant, Abigail Rook. An evil king is turning ancient tensions into modern strife, using a blend of magic and technology to push Earth and the Otherworld into a mortal competition.…