I am about to embark on my seventh novel. (Five books are currently in the marketplace, number six has been entered in the Malice Domestic competition.)
As you may know, I write historical mysteries that are solved today with modern science (had to combine my science museum background with my love of history!) I’ve been often asked how I choose the topics for my book and the simple answer is this. I select a time period and a real event in history to construct a mystery around. In earlier books, I’ve used the Civil War, the Salem witch trials, the Nazi confiscation of art, and the tragic fire at the Triangle Shirtwaist Factory in New York City in 1911, as backdrops.
The modern story lines utilize current technology to resolve the ancient crimes: digital photography, arson forensics, scientific techniques for art authentication, and questioned document analysis, are examples.
For my next mystery, I take the reader back to the Spanish Inquisition, a turbulent time in world history, where heretics were forced to convert to Christianity or exiled from Spain and Portugal.
My main character will be Frank Mead, a New York City homicide detective who has appeared in each of my books. He will have a new romantic interest, Rachel Bejarano, a research librarian at the NYC Public Library, who is on a quest to track down a mysterious necklace that is left to her by her Sephardic ancestors. (Rachel appears briefly in book six, Hart of Madness.) Together they trace her ancestors to a small town in Spain (Cordoba, perhaps?) and the ancient Jewish quarter.
However, murder and mayhem stalks them every step of the way, from Madrid, where they start their investigation, to the glorious palace, Alhambra, in Granada. 
I’m sure you are chuckling as you read this, thinking, “Ahh, the writer gets to take a trip to Spain.” Indeed. Ain’t it grand?
Now the work begins:
Create the historic and modern story lines.
Draw the character sketches.
Research, research, research the locations, the history, the authentic characters of the time, the language, the food, the clothing, et al of 15th Century Spain.
For me, this is the most exciting time in the writing process: molding the essence of an idea into a rich and dramatic story. Writers, you know exactly what I mean.
I welcome ideas and thoughts about your process.