When Christine Riccio was a teenager growing up in New Jersey, she and her sister would upload videos to YouTube of the two of them being silly, dancing to Britney Spears’s “Piece of Me” or attempting a back flip. It wasn’t until Ms. Riccio was in college in 2010 that she “actually talked to the camera” for the first time and decided to upload a video book review of Suzanne Collins’s “The Hunger Games.”

“I was reading a lot of books, and I had no one to discuss them with,” she said, explaining why she turned to the internet. “I was like, ‘I’ll be lucky if I ever get 500 subscribers over here.’”

Initially, Ms. Riccio split her content onto two channels, one for comedy, and another for books. But after college, while interning at Will Ferrell’s production company in California in 2012, she — like many interns — had a lot of free time on her hands. She read and came up with video ideas for her book channel, PolandBananasBooks, and began uploading skits, reactions to book-to-movie adaptations and book hauls (in internet parlance, a haul is when someone shares the items they’ve bought during a shopping spree). Her book channel grew from less than 1,000 to 5,000 subscribers that summer. Now at the age of 27, with close to 400,000 subscribers, she is YouTube’s most popular “BookTuber,” chronicling books for a largely millennial and teenage audience.