Apparently, Calibre is supposed to do this, but I'm not sure how to go about this.
I got locked out of my blog. I tried to use Scrapbook to retreive my posts, but I don't know if I retrieved everything before my ip address got blocked from the site. I filtered out posts from other domains. A lot of the html and css files are numbered. I don't think I can manually organize or sift through all these files. I hope that if there's some automatic way to convert them to one epub file, then I can check that.
These instructions say to go into the editor and add the files from there, but I can't go into the editor until I already have at least one file in the book:
https://manual.calibre-ebook.com/edit.html