Not Book of September, as that accolade is taken, but I'm greatly enjoying the steampunk adventure 'Glass Books of the Dream Eaters' by GW Dahlquist.
Steampunk (if you don't know it) is things set in a Victorian world but with more modern, weird technology.
Picture shows the three main characters: Celeste Temple; a jilted woman in search of revenge, Cardinal Chang; a professional assassin and Dr Svenson;...a doctor. I won't go into the plot here as it is fiendishly complex, but is riproaring fun. Even better, part of a series of three!