Hawthorn by Carol Goodman
One thing that I really liked about this book, or even just the series, is it's connections to real things that happened in the early 20th century. For example, the Triangle Shirtwaist Factory fire is incorporated heavily in the first book and the sinking of the Titanic heavily influences the story in the second one. Now the one I just read, the third one, is really interesting because it's set in the time leading up to and during WWI and talks about how the main characters affected these events. For example, without major spoilers, one of the main characters friends talks about foiling the first few plans to assassinate the archduke Franz Ferdinand only to fail on the third or fourth try. It is connections like these to the real world that I think make the book really interesting. Thanks for your time!