There are some
guidelines. From them:
4. If you are considering creating a particular eBook, please check before you start whether or not that book has already been uploaded to MobileRead (this can be done by clicking the "E-Books" link on the blue bar near the top of the screen, selecting the "Browse Latest Uploads" option, then selecting the "Search E-Books" link and entering either the title or the author into the search box). If you find that the book has already been uploaded,
you are very welcome to upload a new version provided that it "adds" something different to the version already present - this might be a better layout, a new format, significant proof-reading and correction, proper dashes or "curly quotes", for example. If this is the case, when you upload your new version,
please add a link to the earlier version (or simply say who the uploader was),
and state in the description of your book how it differs from the earlier version.