I find prequels, sequels, and sidequels produced by BH & KA, generally, of low quality, closer to YA, soap operas, or Star Wars former Extended Universe, than to Frank's deep philosophical treatises behind the space opera mask. Of all authorized and unauthorized additions to canonical six books, I'd recommend The Dune Encyclopedia edited by McNelly, with Herbert himself's foreword and blessings (though it contradicts Heretics of Dune and Chapterhouse: Dune plotline, because Encyclopedists worked before those were composed), I, Scytale by Edward St. Boniface, Dune: Advent by Marco van Leeuwen, and Dune Elixir novel by Alan Sullivan. But stay afar from low-fat Brian and Kevin's menu.