"Ebook" is a fairly fuzzy term--does it apply to just any electronic presentation of text, or is there a more specialized usage? Is there any feeling from the community that we should be restricting questions and answers to just a few formats, and if so, which ones?
I think the real issue is not just the file formats or the fact that it's in an electronic form; it's also important what content it contains. For example, plain HTML may be on-topic if it's being used to format and present a lengthy work, but not if it's being used for basic web page development. On the other hand, EPUB files are basically always on topic. Likewise,
.txt is off-topic for saving a grocery list, but on-topic when used for markdown, LaTeX, or something similar.
In other words, I don't think we can have a hard-and-fast rule that says some file types are always okay, though we can obviously exclude some (like, say,
All file formats that are used to read books:
- image formats, especially the multipage ones like TIFF and DjVu. Possible annoted with text as with OCR-ed DjVu.
- text formats (possible with images) like EPUB, Mobi.
- formats like PDF used for text as well as images only (however unsuitable for that).
We could of course require that the question actually relates to reading ebooks, but e.g. a recent question on splitting DjVu could be as valid an ebook reader related question as is a question about the necessity of limiting the size of individual HTML parts in an EPUB.
I thin we should not worry too much that the former could be used for completely non-reading purposes as well.
Personally, I tend to use availability rather than functionality as my metric for determining what is an ebook format: it's not that epub and mobi files are reflowable (generally) while djvus aren't; it's that you can't sell djvus through the major ebook retailers, while you can sell epub and mobi there.