Well, for errors, typos, etc. the first thing to try is contacting the person who did the original upload (by PM, usually). That way he/she can correct the source files and upload updated versions in the original threads, for all formats.
For other stylistic changes, contacting the uploader is also advised. I don't think anyone here would have any problem with you (or anyone else) posting "modified" versions of the books (they are public domain, after all), but the original uploader might have some preferences: he/she may want to keep a copy of your files to update if further corrections are necessary, or may wish your files to be uploaded in the same thread or in a different one, or may provide you a better source text...
PS. If you are going to improve the book, I'd suggest converting the straight quotes and apostrophes to "curly" ones. But it's a daunting task, they have to be checked almost one by one, or it's not worth the trouble: better leave them all straight than have ‘em pointing the wrong side.
Last edited by Jellby; 02-26-2011 at 11:47 AM.