The MobileRead library is more a repository of ebooks than a common library. Books from here are not borrowed, but just downloaded and usable with no deadline or restrictions. They are also DRM free, you can use it and copy it to any device or computer, as long as it reads the format.
But, since this library is free for anyone to use, we can only host books that are also free of copyright (or donated by their copyright holders). That means you can upload a book only if you are the author or if the book is public domain (in Canada, or in the US if you upload to the US server). In general, uploading a book you purchased is not allowed.