Pratchett is parodying Hollywood - even more transparently than usual, with the area in question being "Holy Wood." Alchemists (not, it's important to note, wizards) create "the clicks," silent moving pictures. And this slowly allows a weakening of the boundary between the Discworld and other dimensions. Some of the parodies of particular movies and actors were mildly amusing, but not a great book.
I'm most of the way through the next one as I write this: Reaper Man, which I'm finding far superior.