How to Translate a Book — or Just One Chapter — with Bitcoin Lightning
Upload an EPUB. The page reads its table of contents, lists every chapter, and prices only the ones you keep. You get an EPUB back, with its formatting where you left it.
For Humans
Upload, untick, pay, download.
Every book translation service prices a book. Agencies quote per word and take weeks. Software subscriptions want a monthly plan and an account before they will look at your file. Both assume the thing you want is the whole book.
Often it is not. It is chapter nine of a technical manual, the one part of a thesis your supervisor reads, a single section a client asked about, or the two chapters of a textbook that cover this term's syllabus. Paying for four hundred pages to read eighteen of them is the normal outcome, and it is the reason people paste chapters into a chat window instead — losing the headings, the footnotes, the images and the structure on the way in, then rebuilding them by hand on the way out.
The chapter list comes from the book
When you upload an EPUB we read its own table of contents. That matters more than it sounds: most books do not store one chapter per file. A book we tested ships 41 chapters inside 5 files, and its table of contents addresses them by anchor. Listing the files would show you five rows. Listing the chapters shows you all of them, each with its character count, so you can see that chapter nine is 18,000 characters of a 345,000 character book before you decide.
Front matter and reference material start unticked, because almost nobody wants to pay to translate a copyright page, a table of contents or a forty-page index. Tick them back on if you do. The price updates as you choose, and it is the server's price — the number you see is the number the invoice is made from.
Researchers and students
Translate the two chapters on your topic instead of the monograph. The citations and footnote markers survive, so the text stays quotable.
Authors testing a market
Translate a sample — the foreword and the first two chapters — before committing to a full edition in a language you cannot yet read.
Technical documentation
Manuals ship as EPUB more often than people expect. Translate the install and troubleshooting chapters, leave the API appendix in English on purpose.
Reading in a second language
Translate a chapter at a time as you go, and keep the rest in the original. The file stays a single valid book your e-reader already understands.
You get a book back, not a wall of text
The output is an EPUB built from your EPUB. Headings stay headings, italics stay italics, links keep their targets, images and their captions stay in place, and the chapter structure is untouched. The book's language metadata is retargeted so an e-reader hyphenates and reads it correctly, and the table of contents follows the chapters you actually translated.
Two behaviours are worth stating plainly, because they are choices and not accidents. Chapters you leave unticked stay in the original language inside the same file — removing them would leave a book whose contents point at pages that no longer exist. And if a chapter comes back from the engine with its markup mangled, the job fails and issues a refund rather than handing you a half-translated book.
What it costs
Pricing is per visible character, on the engine your target language routes to, with a minimum of 50 sats. Tags, attributes and image references are not billed — you pay for words a reader sees.
Because the engine is chosen per language, the rate differs between, say, Danish and French. The languages page lists every language, the engine serving it, its measured chrF score and its rate. A single chapter of a long book lands near the minimum; the whole book costs what the whole book costs, and you see both numbers before paying.
For Agents
L402 over HTTP, or MCP as translate_epub.
The endpoint takes a base64 EPUB and a target language, answers 402 with an invoice, and returns a job you poll. The quote is a local parse — we never call a translation provider before you have paid.
# 1. Ask for the price. The 402 carries the invoice and the character count.
curl -sS -X POST https://sats4ai.com/api/l402/translate-epub \
-H 'Content-Type: application/json' \
-d '{"epubBase64":"<base64>","targetLanguage":"Spanish"}'
# 2. Pay it, then repeat the call with the macaroon and preimage.
curl -sS -X POST https://sats4ai.com/api/l402/translate-epub \
-H 'Authorization: L402 <macaroon>:<preimage>' \
-H 'Content-Type: application/json' \
-d '{"epubBase64":"<base64>","targetLanguage":"Spanish"}'
# -> 202 with a poll_url; the finished EPUB arrives as a signed download link.One honest limitation: chapter selection is web-only. Over L402 and MCP you buy the whole book. Sending a skip parameter to the L402 endpoint gets a free 406 naming the web page, because silently billing you for a whole book while you believed you had excluded half of it is worse than a refusal.
Limits
- EPUB only, up to 50 MB and 3,000,000 visible characters.
- For a PDF or a Word file, run file conversion first.
- The finished file is a signed download that expires; fetch it when the job completes.
Related
- Turn the same EPUB into an audiobook — chapter selection works there too, and it can translate before narrating.
- The languages AI does not speak — why the engine is chosen per language, and how each one is measured.