I can confirm that cortosis ore appears in Jedi Council: Acts of War:
Adi Gallia: I don't understand... what's a "cortosis shield"?
Micah Giett: Contact with cortosis ore will cause a lightsaber to fail. By the time one realizes the cause of the failure... But cortosis ore is very rare. And now that we know of the shields, we have little to fear.
(Jedi Council: Acts of War, collected in Omnibus: Rise of the Sith, page 142)
However, as DavidW points out in the comments, cortosis itself is older, appearing at least as early as the 1998 novel I, Jedi:
They started with a basic armor shell and covered it with spun cortosis ore fibers, providing them some modicum of protection against all sorts of weapons.
(I, Jedi, chapter 47, page 530 of 1999 paperback edition)
Later in the novel, Corran's lightsaber fails when hitting the armor. Vision of the Future, published later in 1998, references this incident:
"There must be cortosis ore in this rock," he told her. He held his glow rod up to the rock face, the light dancing off tiny sparkles.
Mara shook her head. "Never heard of it."
"It's apparently fairly rare," Luke said. "All I really know about it is that it shuts down lightsabers. Corran and I ran into some Force-users once who'd made sets of body armor out of woven cortosis fibers. It was quite a surprise."
"I'll bet," Mara said, a memory of her own drifting up. "So that's what the slab of rock was Palpatine had between the double walls of his private residence."
(Vision of the Future, page 211 of 2011 edition, via Google Books)
The concept evolved for its later appearances in video games to only allow weapons and armor made with cortosis to deflect lightsaber strikes without disabling the lightsaber.