In Japan, there are numerous illustrated field guides (図鑑 or "zukan" in Japanese). Many of them are for kids and provide illustrated looks at things like dinosaurs, bugs, or trains. Then, there are zukan that are drawings guides—drawing guides for adults.
These adult zukan feature women (or men) in various nude poses. Many of them claim to be artist desk references and not all of them feature nudity (check this gun zukan with improper weapon handling).