The trouble is that if you want the complete works of a public domain author then they aren't always available in html or text files.
I wanted to add to our collection of Anna Katharine Green. Harry and I have done all the ones on Project Gutenberg. Now I'm getting some more novels from the scans at The Internet Archive. They all need extensive reformatting and checking, as RWood points out.
It is worth checking the Distributed Proofreaders' lists of works in progress, so as not to start work on a scan, spend days on it, then find that they are about to upload a nice clean copy.