I certainly enjoyed ‘The Art Trilogy’. Quintessential Barker; sublime, mind-bending and horrifying.
Loner mystics (à la Theodore Wicker) venturing to alternate planes of existence, and super-human battles that determine the fate of the world.
My favourite Barker novel ‘Weaveworld’ may be not as epic or complicated a ‘The Art Trilogy’, but perhaps therein lies its charm and ability to enchant readers. Very much about a Secret World.
It also has Cenobites.
