Hey there -- I do volunteer support at Open Library and we're aware of the issue on our end. Some people seem to be able to download books, some people can download only some books and some people (myself included) can't download anything at all. We usually have a few people who have downloading problems so it took us a while--and some very helpful patrons who walked us through their problems--to narrow it all down. Now that we know what is happening, at least loosely, we have to figure out how to fix it. We're a pretty bare-bones operation so it's been a challenge to try to see patterns and figure out what needs doing.
Sorry I know this isn't helpful really, but wanted you to know we're aware of it and we're as bummed as you are.