Restoring an old book - GIF

[Inner]

Are the leaves made of vellum? Since she immersed it in a liquid, I didn't think they were paper.

Yes, paper pages/leaves. Full immersion in water is very common for cleaning or removal. The only catch is that first you need to test any inks on the paper to see if they're water soluble. If they are soluble (they usually are not), you'll need a different procedure.