How much research should go into a historical romance or novel? How much to include of all the hours you have poured over books and visited museums getting information? How much do you need to put in for your book to strike an authentic note?
Only as much as you need on a bare basis.
Research is done for the author, not the reader. If you include a few items of authentic historical accuracy, you will make them happy. If you include every nuance of every detail you learned while researching the book, you will make them stop reading.
They are reading your book for entertainment, not really education. At least, a little interesting facts will edify them properly, but not bore them to tears.
For example, if I wanted to put my characters on a train with a famous historical figure, I would need to research when that real person took a train in their public life and when it could be woven into the story, or if it even could. Would it work? Would the whole timeline of the book be thrown off if I created this scene for the sake of historical info?
If it will work, how much of it do I include? I personally would only show the famous person in a cameo appearance and let that be it. That is so the reader knows I did my research on the time, I made train travel seem viable, and I threw in some history for them to fill in the world building.
What do you do with your research?