I've just passed page 666 in "The Illuminatus! Trilogy" and was expecting something apocalyptic or at least interesting , but barring Moses crossing the Red Sea there wasn't too much out of the ordinary barring a reanimated Nazi army rising from the bottom of a lake near a rock festival close to Ingolstadt which is fairly normal for that book.
I can only think of two other books that I've read have a page 666 that's "Imajica" and "Lord of The Rings" and maybe of of Michael Moorcock's Corum collections. I suppose The Bible and Quran also have one.
666 is defined as "The Number of The Beast" and features in many biblical and occult writings and films. as well as being an upside down 999. Aleister Crowley modelled himself as The Great Beast and many metal bands such as Led Zeppelin and Iron Maiden , and jazz artists such as Graham Bond tapped into his legacy.
I remember visiting a youth club as a teenager and we brought records and one guy brought the "banned" "666" by Aphrodite's Child (featuring Vangelis and Demis Roussos" and I was well impressed with a lot of the album and have a copy in by collection. "Babylon" and "The Four Horsemen" are well worth tracking down. Click on the name for Youtube links. But we'll go with the more obvious Iron Maiden rampage on this Saturday morning.