I went digging on this, because I thought you were wrong.

Turns out the official book name is indeed singular, and in fact I tracked this singular usage all the way back to Athanasius 39th festal letter in AD 367, listing the accepted books of the New Testament - "the Revelation of John".

"The Revelation" is also the first word of the book in greek. (Ἀποκάλυψις or apokalypsis)

So thanks for the enjoyable rabbit trail.