Hallowmere Series by Tiffany Trant

Does anyone remember the Hallowmere series? I read the first two way back when I was in middle or high school. I can’t quite remember only that it was more than a decade or so. I remember it being about dark fairies and there were supposed to be 6 in the series. I looked them up and can’t find information about them. Amazon has them but you can only buy them used, and theres no information about the 6th one. Does anyone know anything more about them?