# boko
[](https://github.com/zacharydenton/boko/actions/workflows/ci.yml)
[](https://crates.io/crates/boko)
[](https://docs.rs/boko)
[](LICENSE)
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
| KFX | yes | yes |
| AZW3 | yes | yes |
| EPUB 2/3 | yes | yes |
| MOBI | yes | no |
| Markdown | no | yes |
| Plain text | no | yes |
## 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
```rust
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](https://www.libvips.org/)) 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](https://github.com/rust-fuzz/cargo-fuzz)):
cargo +nightly fuzz run from_bytes
## Credits
- [KFX Input/Output plugins](https://www.mobileread.com/forums/showthread.php?t=291290) by John Howell (jhowell) — kfxlib powers `tools/kfxcheck.py` and served as the KFX conformance reference
- [calibre](https://calibre-ebook.com/) by Kovid Goyal and contributors
- [epubcheck](https://github.com/w3c/epubcheck) by the DAISY Consortium / W3C
- [Standard Ebooks](https://standardebooks.org/) — public-domain test fixtures
## License
[GPL-3.0-or-later](LICENSE).