Predictions for the Great Schools Books
Brian mentioned these titles in an interview last August: The Sisterhood of Dune, The Mentats of Dune, The Swordmasters of Dune. They've also mouthfarted in the past about a Guild book and a book on the C.E.T. & OCB (see TheKJA's reply to Carlos yesterday), but those would probably be absorbed into the main three books, especially if Brian is serious about putting a cap on "major books" in the near future.
I've already put something under the C.E.T. heading, about that ole "neutral island".
