I have a Plug-in in the Calibre Editor, but the ebook is maybe 1000 files? It seems to only process the open files, not all the files.
Ben Franklin's Harvard Classics + a bunch of other books making a 52 MiByte file! I've fixed a lot of garbage in the CSS and it then combined a bunch. But it seems to use a load of pointless extra divs and spans. It seems to have been created by someone with an enthusiasm to exactly duplicate a print experience using Sigil. Not Sigil's fault!
Probably I'd have been better off collecting all the titles of gutenberg myself