Looks like it's not just mystery and thriller collections that are down to £3.99 at Amazon UK.
Dune: The Gateway Collection - Frank Herbert
Dune / Dune Messiah / Children of Dune / God Emperor of Dune / Heretics of Dune / Chapter House Dune
The Shadow Saga Omnibus - Orson Scott Card
Ender's Shadow / Shadow of the Hegemon / Shadow Puppets / Shadow of the Giant
The Collected Joe Abercrombie - Joe Abercrombie
The Blade Itself / Before They are Hanged / Last Argument of Kings / Best Served Cold / The Heroes / Red Country
The First Sookie eBook Collection - Charlaine Harris
Dead Until Dark / Living Dead in Dallas / Club Dead / Dead to the World / Dead as a Doornail / Definitely Dead
The Gollancz Collection - Robert Rankin
The Hollow Chocolate Bunnies of the Apocalypse / The Witches of Chiswick / Knees Up Mother Earth / The Brightonomicon / The Toyminator / The Da-da-de-da-da Code / Necrophenia / Retromancer