There are 3 books in the Plunkett and O'Nelligan series.
The Plunkett and O'Nelligan series does not have a new book coming out soon. The latest book, The Haunting Ballad (Book 2), was published in October 2014.
The first book in the Plunkett and O'Nelligan series, The Seance Society, was published in October 2013.
The Plunkett and O'Nelligan series primarily falls into the Historical Mystery and Private Investigator genres.