If you're asking about the physical format of your book (e.g. paperback, hardcover, etc.), the answer will depend on your publisher's preferences. If you're self-publishing, you'll have more control over the format, so you can choose whichever you prefer.
If you're asking about the file format of your book (e.g. Word, PDF, etc.), again, it will depend on your publisher's preferences. If you're self-publishing, you can choose any file format you like, as long as it's a standard format that can be easily converted to other formats if necessary. For example, you might want to submit your book as a Word document so that it can be edited easily, but then convert it to a PDF for distribution.
