The short answer is probably around August next year.
The long answer is Terror in Topaz hit a stumbling block and for reasons I won't go into, there has been a considerable delay on the administrative end. This flowed into a delay on the creative end ie... I still have to finish writing the book and I'm not a very fast writer.
So my profuse apologies to those anxious readers who are chomping at the bit to see how the series ends (yes, Book 4 will be the last). Having said that I am not saying goodbye to the characters forever... there may be other stand-alone stories in the wind!
In the meantime, you are very welcome to peruse my backlist of Alison Stuart titles... there are mysteries mixed into the historical (Lord Somerton's Heir, Gather the Bones and The Goldminer's Sister and coming in January, The Homecoming) and if you have not already signed up to my email list, that is the best place to get updates on Harriet and her friends AND you get a free download of Harriet's prequel - THE UMBRELLA.
Now I better get my head down and finish that book!