If anyone is interested in a Python script and some CSS files that turn an AO3 html download into a PDF file that looks like a regular book, I have put together a package to do so.
I used them on my "The Watcher on the Water" work, and here are some samples of the output.
Title page:
Table of Contents (I had to fill in the page numbers by hand, but the format is all generated by Python and CSS):
Main text with chapter heading, justified, single-spaced lines, and first line indented:
The Table of Contents has pages numbered with lower-case Roman numerals, and the main text has regular Arabic numerals. There's code for a cover page, too, but that's just an image. Everything is editable, obviously.
It can handle works with chapters, works without chapters, and a list of works like from a series. I've just converted my own works, so it's possible there are quirks that I didn't encounter. If so, just let me know and I'll try to handle them.
You can find the files in a zip file here, I hope.
https://drive.google.com/file/d/1o5QxA46WBMD4O4COgc1X3fBETI0Zq5a0/view?usp=drive_linkI uploaded the Toivosaari PDF separately in case anyone wants it.
https://drive.google.com/file/d/1V1lpu454dwVLmx5ne_ijXY7K78bvjLrU/view?usp=drive_link