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FictionBook is the e-book format the English-speaking world never heard of but Russian-speaking readers know intimately. The format stores book content as structured XML — body text, footnotes, images, table of contents — all in a single self-contained file. It's semantic by design: the format describes what the content is (a chapter, an epigraph, a poem) rather than how it should look.
The format dominates Russian-language digital publishing. Libraries like Flibusta and LitRes built their catalogues around FB2. Russian e-ink readers (PocketBook, ONYX BOOX) treat it as a first-class format. The XML structure makes FB2 files easy to convert to other formats — Calibre transforms FB2 to EPUB, MOBI, or PDF with minimal effort.
Outside the Russian-language ecosystem, EPUB has won. But FB2's XML purity has its advantages: files are smaller than EPUB (no ZIP overhead), easier to parse programmatically, and store metadata cleanly. If you encounter an FB2 file, Calibre opens it on any platform and converts it to whatever your reader prefers.