The palm-sized, lavishly illustrated paperbacks known in Japan as “light novels” can have some heavy titles. “That Time I Got Reincarnated as a Slime” is one. Another, “Is it Wrong to Try to Pick up Girls in a Dungeon?” is a bestseller, but so is “I Want to Eat Your Pancreas.” And that’s a love story. Of course.

They can also have some heavy political repercussions. Earlier this month, a light novel called “(New Life +) Young Again in Another World” had its big-budget anime adaptation and future publications in all languages abruptly canceled after its author, pen name Mine, was found to have posted racist tweets denigrating Chinese and Koreans. The offensive posts, first issued four years ago, were deleted, and Mine publicly apologized. But the tweets and the novel’s storyline, in which a Japanese swordsman who murdered 3,000 in China is reborn in a land of monsters, were not taken lightly.

Quite what qualifies as a light novel is a subject of debate and ongoing revision. According to translator and journalist Kim Morrissy’s 2016 Anime News Network article, Keita Kamikita, manager of an online fantasy and science fiction forum, coined the term in 1990 when he noticed that new types of fantasy prose narratives were drawing the attention of manga and anime fans. This burgeoning readership was not entirely young, so the “young adult” label didn’t apply. Nor were they reading works that adhered to pre-existing genres such as romance, mystery, horror and so on.

The new format was essentially a mashup, incorporating elements from anime, manga, video games, fan art and fan fiction. It was character and dialogue-driven, replete with provocative illustrations and heavily reliant upon the viral energy of the internet, where many of the stories get their start. Veteran publisher Kadokawa Shoten eyed opportunity and quickly cashed in on what they called the “media mix” phenomenon, issuing most of the books in cheap bunkobon (portable paperback) format and churning them out, releasing as many as six per day, reports industry journal Shuppan Geppo. Light novels with the right tie-ins with popular anime, manga, movies, game or TV series sell in the millions.

Or at least they used to. Like manga and just about every other printed media, light novel sales have slipped in the domestic market since their peak six years ago. According to industry data, in 2012, they brought in ¥28.4 billion and comprised 21 percent of all paperback book sales. Last year, those numbers fell to ¥19 billion and 18.7 percent, continuing a downward trend.

Nevertheless, publishers are still flooding the shelves. A recent article in Toyokeizai notes that twice as many light novels were printed last year compared to 10 years ago — and while print keeps falling, digital sales spiked 22 percent in 2016 alone.

As with manga, light novel publishers are also looking abroad for help. Two years ago, Kadokawa added Thailand to its list of foreign publishing investments, which already includes Taiwan, Hong Kong, China, Malaysia and the United States. And there are signs that English-language readers are slowly catching on, if not yet fully catching up.

The key light novel title that helped drive Japanese sales in 2012 was Reki Kawahara’s “Sword Art Online” (“SAO”), a series about multiple game-driven virtual reality worlds whose media mix spinoffs went manic. The books were first published in 2009. Subsequent years saw a second series of novels, eight manga adaptations, numerous television and feature-length anime productions, six video games and a forthcoming live action series produced by Netflix. The light novels have reportedly sold more than 20 million copies worldwide.

North American publishers Seven Seas Entertainment, Vertical Inc., Viz Media and Yen Press have all been rapidly expanding their rosters of light novel translations. Yen Press, a joint venture between Kadokawa and the New York-based Hachette Book Group, has led the pack in publishing and promoting light novels in English-language markets, and founding editorial director Kurt Hassler sounds like he’s just getting started.

Hassler launched light novel imprint Yen On in 2014, introducing Kawahara’s “SAO” with the modest goal of publishing 12 books annually. That figure doubled the following year, and now Hassler tells me that Yen On will release 110 light novels through the rest of this year, representing growth of nearly 1000 percent in a mere four years.

“It’s been exponential,” he admits, adding that today, the light novel versions of stories sometimes outsell their manga counterparts. “Light novels have very quickly become the cornerstone of our business.”

Hassler points out that the digital format of e-books is especially popular with light novel readers, affording easy and immediate access for those hooked on a serial narrative connected to an anime or manga series. That caught the attention of Sam Pinansky, an American entrepreneur with years of experience in the anime business, having worked at Tezuka Productions and Yomiuri Television Enterprise. Two years ago, Pinansky started a digital English-language light novel publishing company called J-Novel Club, single-handedly designing the site, licensing titles from Japanese publishers and amassing a team of translators.

Pinansky’s e-book strategy is to get the titles translated and posted online fast — an effort, he says, to stay ahead of the “fanlations” (unlicensed translations pirated and posted online), and to keep his members coming back for the latest episodes. The site posts a new title nearly every day, and sometimes up to six titles at once.

Pinansky and other light novel publishers will descend on Los Angeles next week for Anime Expo 2018, the largest Japanese pop culture convention in North America, to unveil their latest products before 100,000 attendees. Tokyo-based translator Emily Balistrieri believes that those anime and manga fans are missing out if they don’t also read light novels.

“You can have just one of these experiences, but if you have each one, they build on each other and you can appreciate different aspects of the series,” she says. “I can’t believe I didn’t realize it sooner.” To quote the great Pokemon: You gotta catch ’em all.

Roland Kelts is the author of “Japanamerica: How Japanese Pop Culture has Invaded the U.S.” and a visiting lecturer at Waseda University.