“The students who come into the fields of Japanese literature and folklore as undergraduates are heavily influenced by popular culture,” Mr. Foster, a folklore professor at Indiana University and author of “Pandemonium and Parade: Japanese Monsters and the Culture of Yokai,” said. “They grow up with these things through anime and manga and want to know where they come from.”

Stories about yokai have been popular in Japan for centuries, from the 11th-century classic “The Tale of Genji,” in which they’re called mononoke, or “mysterious things,” to contemporary anime series. The yokai themselves are everywhere in Japan, in films and cartoons, on billboards and even on beer bottle labels. The latest yokai craze began in the 1980s and has been going strong ever since, part of a long history of booms that dates back to the Edo period (1603-1868). Last year, “Yo-Kai Watch” was the top-selling video game in Japan, and there are plans to release the game in the United States this year.

Image A ghost depicted on an 18th-century scroll. Credit... UC Berkeley Art Museum and Pacific Film Archive

Relatively few of the thousands of texts and scholarly studies about yokai have been translated from Japanese, which makes these latest books all the more valuable to nonfluent seekers of the original tales. In “The Book of Yokai,” Mr. Foster draws from texts and folk tales dating back to Japan’s Heian period, from the works of the 10th-century writer Abe no Seimei (a midlevel bureaucrat who has been reborn in contemporary manga and anime as a young, beautifully androgynous sorcerer) to the tales of the early-20th-century scholar and avid story collector Kunio Yanagita, considered one of the founders of Japanese folklore studies.

New texts and stories are still being discovered and translated, and the abundance of source material can be a blessing and a curse for yokai researchers. How do you define a creature that can vary from period to period, or even town to town? “When I see yokai mentioned, it will often just say ‘a kappa is a so-and-so,’ ” Mr. Foster said. “So my responsibility is really to complicate that, so that people will understand that a kappa can be many different things, depending on where and when you’re speaking of it.”

There are also beasts whose stories have been lost, but whose images remain, like the tofu-kozo, a bigheaded servant boy holding a block of uncooked tofu. “There’s a number of images of that, but nobody knows why they exist,” Mr. Foster said. “It might have been an Edo period advertising campaign, but that’s all speculation.”