While manga is what many think of in terms of popular Japanese literature, there's another medium that's almost as massive: light novels. Many popular anime got their start as light novels, and because of that, the form is getting bigger than ever. My Next Life As a Villainess, That Time I got Reincarnated as a Slime, Sword Art Online, the list goes on. But light novels aren't just big in Japan anymore, it's actually easier than ever for fans overseas to get into them.
Light novels got their start around 1975 when magazines that published short stories for young adults began to include illustrations before each story and published articles about pop culture like anime, manga and video games. Eventually, these companies would sell these stories in small volumes, giving rise to light novels. Author turnaround is high for light novels, with publishing houses holding annual contests to find new authors to publish.
It's also expected that they consistently churn out new stories, the most extreme example of this being Kazuma Kamachi, author of A Certain Magical Index, who wrote a new novel every month for two years. In their search for new authors, many companies turned to the internet and offered deals to popular authors who published their stories online. Sword Art Online and That Time I got Reincarnated as a Slime actually started out as web novels before being picked up for publication.
Light novels are profoundly small, with the average being A6 size, or about four by six inches. Word count averages out at about 50,000 words, making them very quick reads. While wildly popular in Japan, light novels haven't quite found their footing overseas. Some companies have begun publishing works in English, but they've been slow to catch on, partly due to lack of awareness, but also because anime and manga adaptations of the same material tend to cross over first. But despite this lack of notoriety, light novels are quite readily available for international fans.
There are a number of choices for English readers that prefer to read online, including the J-Novel Club website-app. While it doesn't have the widest selection ever, J-Novel Club has a good selection of popular and classic light novels, like Ascendance of a Bookworm, Otherside Picnic and Slayers and allows subscribers to read novels before they're published. A regular subscription is about $4.95 a month, or $54 a year, while a premium subscription is $10.95 a month and $120 annually.
The other main option for e-readers is to buy the novels individually from Kindle, Nook or Kobo. Kobo is owned by Rakuten, a Japanese online retailer, and almost exclusively sells e-books, meaning they tend to have better rates than anywhere else. Many light novels that are available in English digitally are also available in print, from Amazon, Barnes&Noble and other book retailers that carry manga.
In terms of actual titles, the most popular titles out are actually the best place to start. The most popular genre is isekai, or "different world," often centering around a protagonist that's magically transported to another world where they must survive by learning about the world around them. No Game No Life is one of the best examples of this, with two siblings arriving in a world where games determine one's fate. Overlord follows a gamer who is transported into the world of his favorite video game, where the NPCs have become sentient.
If isekai isn't really your jam, there's also Spice and Wolf, a fantasy story about a traveling merchant who becomes a companion for a lonely, homesick deity named Holo who wants to return to her homeland. The Melancholy of Haruhi Suzumiya is best known for its anime adaptation, but the light novel is where it all started. The story follows high-schooler Kyon as he is dragged into the shenanigans of his classmate Haruhi and her SOS Brigade as they investigate mysterious goings-on, all while Haruhi is observed by various organizations because of her reality-warping abilities.
If horror is more your style, Another is another great series. It tells the story of a transfer student who finds his new classmates ignoring one particular student for apparently no reason. Slowly, classmates being dying of a mysterious "curse" that plagues each new set of students in Class 3-3. Goth is a small anthology of stories that explore the lives of two students who share an interest in gruesome murders, and soon their interest begins to hit a little too close to home. Whatever genre you're into, light novels offer something for everyone.