There are loads of legitimate reasons a reader might dislike “The Ink Black Heart,” the sixth book in the Cormoran Strike series. Writing as Robert Galbraith, J.K. Rowling has spun out a 1,024-page-long mystery, which is long even by, say, Elizabeth George standards and at least 500 pages longer than the story warrants. Far too many of those pages are filled with tweets; characters discussing those tweets; transcripts of website interviews; and gamer group chats complete with all the concurrent private chats they spawn. These last are presented in column form; they are challenging to read and …