Upload a scan to Wikimedia Commons. The text must no longer be under copyright in the United States and in the country of origin or freely licensed works to be allowed on Wikimedia Commons. If it is in the public domain in the United States, but not in the country of origin, you should upload the file to Wikisource instead.
You can also use URL2Commons or IA Upload tools. The latter uploads DjVu files to Commons from the Internet Archive, and is especially recommended if the work you want to add is already on IA or if you upload it there first.