I have released 17 non-fiction kindle books in two different general niches/categories. One category, all the books tanked and did next to nothing. The other category, the majority of the books did ok but not great, except two of them are doing very well. They both belong to the same general topic/category, but are about different things. I'm trying to decipher what it is about these two particular books that's making them sell so much better than my others so I can replicate whatever I did right, but I can't see anything obvious sticking out at me that would make them better sellers than the other books in their same category.