It’s “Book of Revelation”. Singular. Probably because it talks about Jesus being revealed to the whole world. He will be revealed once and for all, so singular.
Regarding Revelation being behind us. It will be as clear as thunder visible from the east to the west and many cities will be destroyed etc what didn’t happen yet I’m sure. Bible warned that people will try to convince us that it has passed already. Quoting KJV from 1611 because I like the used language and find it a clear text.
2 Thess 2, “That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand. Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;”