UTF8 preferred, since I'd be doing mostly Japanese. Shift-JIS would be acceptable. Basically just plain text. For books, anyhow. If I sent any comics, I'd want image files.
Though, after thinking about the cost of shipping, book, etc, I'm not sure I'd send much... It'd be only things that I really, really want to read and just haven't learned the vocab for yet. And there really isn't much of that.
They OCR it, so with the PDF you get whatever text data they are able to extract from it (don't know about the encoding, but that is easy to convert), plus additional information.
djvu + OCR and epub would be my preferred formats. The original tiff along with it would be nice too, in case there's a better lossy format in the future.
