Another entire series I have never managed to figure out the appeal of is Terry Pratchett's novels, although I know he is hugely popular.
I remember years ago, in our local shopping mall, Fred Truman was signing copies of his autobiography in WH Smiths on the second floor, and Terry Pratchett was signing his latest Discworld novel at the Waterstones two floors down.
I was in Smiths for about twenty minutes, during which time one lady bought FT's book and got it signed, and when I headed past the queue at Waterstone's, it was about three hundred people and growing fast out of the doors and down the street!