For me, research is really part of the fun in writing something with history involved. I love being able to make what I see in my mind come alive, and being able to tie it to something that actually happened is amazing.
I have to say that with historically involved fiction research is a constant and ongoing process. It does not end, because as the story evolves you have to be able to keep it going and you need information to do that. So your mind is constantly spinning and things are constantly growing. And it is absolutely not true that you can find anything on the internet.
When it comes to Countess Bathory there was not a lot of information to begin with, and not very much in English. Raymond T McNally's book 'Dracula Was A Woman', at one time the book about the Countess, was released in 1984 (for a little perspective, I was two years old) and I was desperate for something more current. I found some of what I needed in Tony Thorne's book 'Countess Dracula' and Valentine Penrose's book 'The Bloody Countess', but I still felt like I was missing something.
I started work on Rebirth anyway, and left some blank spaces where I needed more information. And when I was about knee deep in it, I stumbled upon Dr. Kimberley Craft's biography 'Infamous Lady'. Since then she has also released 'The Private Letters of Countess Erzsebet Bathory', and my depiction of the Countess was shaped because of both these books.
Both books include translations of documents that have never been in English, Infamous Lady has trial transcripts and Private Letters has correspondence written in the Countess's own words. Infamous Lady is easy to read, unlike most royal biographies that are chalked full of the politics in the country at the time and very little about the actual person, you get a real sense of who this amazing woman was.
I spent a lot of time thinking about the moment the Countess enters the book. After watching this video of Hilary Mantel, author of Wolf Hall and Bring Up The Bodies, and the amazing historian David Starkey and listening to them talk about the moment Henry VIII enters is really amazing, and I found it inspiring.
After Rebirth, writing the second book was a bit different. I'd developed habits, good and bad, for writing and researching and I fell into a groove and things began to gel rather well. I took on some new challenges for the research aspect, I had them going to new places and experiencing new things that I knew nothing about so I had to give myself a crash course in things like food, customs, and regional changes in Europe at the time. Because borders have changes, place names have changed also.
I have had amazing luck with a series of children's non fiction books published by DK. You know those great kids books that are full of pictures and laid out beautifully? I've used those quite a bit, for small details like names of weapons and clothing and little details that DK books are so amazing for.
I guess what I am getting at is when it comes to research you have to do what feels right for you. Someone may be able to recommend a book to you, but how you go about researching your work is a style all your own. Whatever works for you is exactly how you should do it.