boko

boko is a fast ebook converter for EPUB, KFX, AZW3, and MOBI, written in Rust. It is the only KFX writer that doesn't shell out to Amazon's proprietary Kindle Previewer, so it runs anywhere: natively on Linux, headless on a server, in Docker, or entirely in your browser.

KFX is the preferred format for Kindle as of 2026 — it renders with hyphenation, kerning, and ligatures. AZW3 doesn't. MOBI (Calibre's default Kindle format) is 25 years old at this point.

Browser app: https://zacharydenton.github.io/boko — converts ebooks fully client-side.

Formats

Install

Requires Rust 1.85+.

cargo install boko        # CLI
cargo add boko            # library

CLI

boko convert in.epub out.kfx
boko convert in.epub out.azw3
boko convert in.kfx  out.epub

boko info in.epub
boko info --json in.epub

boko dump in.epub
boko dump -c 0 in.epub

KFX/KDF/Ion internals can be inspected with the kfx-dump subcommand:

boko kfx-dump book.kfx
boko kfx-dump -f metadata -f sections book.kfx

Library

use boko::{Book, Format};
use std::fs::File;

let book = Book::open("in.epub")?;
let mut out = File::create("out.kfx")?;
book.export(Format::Kfx, &mut out)?;

Full API: https://docs.rs/boko

FAQ

How do I convert EPUB to KFX on Linux without Kindle Previewer?

cargo install boko, then boko convert in.epub out.kfx.

How is this different from the Calibre KFX Output plugin?

The Calibre plugin doesn't write KFX itself — it drives Amazon's Kindle Previewer under the hood, so you must install Previewer (on Linux: under Wine, where its GUI doesn't work, and not at all inside Flatpak/Snap containers). boko is an independent KFX writer with no external dependencies.

Can I read KFX files with it too?

Yes — KFX import is supported, so you can convert your existing (DRM-free) .kfx books to EPUB, AZW3, Markdown, or plain text.

Why doesn't my sideloaded book show its cover in the Kindle library?

The Kindle never renders library covers from a sideloaded book itself — it looks up a sidecar image on the device, keyed by the book's metadata. Generate it (here with libvips) and copy it alongside the book:

id=$(boko kfx-dump -f metadata book.kfx | awk '/content_id/ {print $2}')
vipsthumbnail cover.jpg -s 330x500 -o thumbnail_${id}_EBOK_portrait.jpg

where cover.jpg is the book's cover image. The book goes in documents/, the thumbnail in system/thumbnails/.

Architecture

Format → semantic IR → format. Imports compile to an intermediate representation: nodes, computed styles, semantic roles, metadata, TOC. Exporters render IR back out.

EPUB ─┐                    ┌─ EPUB
KFX  ─┼─→  semantic IR  ─→─┼─ KFX
AZW3 ─┤                    ├─ AZW3
MOBI ─┘                    └─ Markdown / text

Contributing

Bug reports with sample files welcome, especially KFX and AZW3 edge cases.

cargo test
cargo clippy -- -D warnings
cargo fmt --check

Fuzzing the import parsers (requires nightly and cargo-fuzz):

cargo +nightly fuzz run from_bytes

Credits

• KFX Input/Output plugins by John Howell (jhowell) — kfxlib powers tools/kfxcheck.py and served as the KFX conformance reference
• calibre by Kovid Goyal and contributors
• epubcheck by the DAISY Consortium / W3C
• Standard Ebooks — public-domain test fixtures

License

GPL-3.0-or-later.