He doesn't say anything but the gratitude is rolling off of him, so much that it might be staggering when it's replaced with an odd blankness. Lucifer instantaneously flits back to his "perch" and carefully selects one of the books from under a stack.

It's relatively thin, looking more like a journal than anything. The notes inside are meticulously written, not pulled forth from the mind, and without any emotion in the rundown. All strictly-stated facts.

Lucifer reappears by Istredd, hands him the book, and takes his seat again.

"Reading material," he explains.

The contents are about the Mark of Cain. It's what Lucifer has told Geralt, including further, minor details here and there, including a drawing of the Mark itself.

If Istredd looks, Lucifer's name is on that first page and might explain his sudden blank thoughts.