The 'nude' comment made Percy chuckle, but it was more out of politeness than humor that he laughed. He knew when he was being patronized, and that he didn't find very funny. Tracey was a friend, however, so he tried not to be offended.
"Oh! That's very interesting. I never really considered how those portraits were made, I suppose I simply took them for granted." Percy turned back to the bookshelf and the many titles there. "I don't think I've ever come across a book like that before. I wonder..." Naturally, he began helping Tracey find what she wanted. "Have you tried speaking to the artists of these portraits? Going directly to the source?"