The Ninth Gate DVD was only $10.99, so I figured, why not? They used a couple of different typefaces for the book — none of which were Caslon Antique, I am very thrilled to say — but were any of them, as I was told, my fonts?
Here's a still from the movie, followed by the same text set in JSL Ancient. Unfortunately, I didn't have the entire text to set; since the lines are fully justified, the interword (and possibly intercharacter) spacing will be different.
Personally, I see enough differences to conclude that the still does not contain JSL Ancient. Specifically, the height of the 't' and 'v', the bow thickness of the 'q', the shape of the 'ct' ligature, the ascender height of 'b' and 'd', the shape and size of the comma and the curve of the 'f'.
So perhaps my email informant was mistaken, or perhaps they used JSL Ancient for other props. Oh well.