What are curious plates in books?

Answer 1

This is a euphemism that usually refers to erotic (often kinky) pictures or engravings, added to books that might or might not otherwise be of an erotic nature, mostly in the 19th century, to increase sales or to appeal to the fetishist market.

Answer 2

Curious plates in books refer to illustrations, diagrams, or images that are unique or unusual in some way, often sparking interest or prompting further exploration. These plates can provide additional context, visual aids, or enhance the reader's understanding of the content presented in the book.