The collection of book related content, creators, and their active audience on TikTok. There's also BookTube (on YouTube), Bookstagram (on Instagram), and every platform has their own version I just don't know the name. It covers reviewers, fans, challenges, charity drives, diy rebinding and book repair, the Regency Romance fanfic community, and basically anything you can think of.

When these are used as adjectives for specific books, it means the book got lots of attention on the platform. Or, more commonly, someone put a sticker or digital label on it for marketing purposes or as criticism because there's a handful of content mentioning the book.