My understanding is that book bestseller lists are essentially just opinions, e.g. NYT has a select list of book stores they contact to gather sales information

My understanding is that book bestseller lists are essentially just opinions, e.g. NYT has a select list of book stores they contact to gather sales information. There are no industry-wide sales metrics, unlike the music industry. From what I've read, this is why you never see Christian literature on the top seller lists. The Christian publishing industry is absolutely massive and would consistently dominate any objective top sellers list.