A book can be in print for decades. The application you wrote it in won't be. That mismatch is the whole of the format question, and it has a clear answer: keep at least one copy in a format that many programs can read, because durability comes from breadth of implementation rather than from any property of the format itself.
By durability
| Format | Durability | Cost |
|---|---|---|
| Plain text | Highest; readable by everything for ever | No formatting at all |
| Markdown | Very high; plain text with conventions | Structure only, no rich layout |
| Common open document formats | High; many implementations | Complex, and well supported |
| EPUB | High; an open standard | A publication format rather than a working one |
| High for appearance | Poor for editing | |
| A single application's project format | Low | None while the application exists |
The bottom row is not an argument against using such tools. It is an argument for keeping a second copy in something from the top of the table, which costs one export.
The practical arrangement
- Work in whatever suits you. Durability is about storage, not about drafting.
- Export to an open format at the end of each draft.
- Keep a plain text copy of the finished text, which is the ultimate fallback.
- Keep the published files as well, since they are open standards.
- Store notes in plain formats, since they are the most often lost.
- Check annually that the copies open.
Do point three even though plain text loses italics. A plain text copy of a novel is a few hundred kilobytes and it will be readable by any computer for the rest of your life.
The reference layer
The Novel Kit exports term data in structured open formats, which means the naming and vocabulary layer of a book can be archived as a small durable file alongside the text. Because the tool stores no manuscript, that export is the artefact.
A practical next step
Export your finished manuscript to plain text and keep it with your archive. It costs a minute and it's the copy that outlives everything.
Sources checked
Frequently asked questions
Which formats last?
Plain text first, then widely implemented open standards. Anything a dozen programs can read will still be readable in twenty years.
Is a common document format safe?
Reasonably. It is an open standard with many implementations, which is what durability actually requires.
What is risky?
A single application's project format, especially from a small vendor, since it is readable by exactly one program.