In c.l.postscript and c.p.printers, Computer Nerd Kev wrote:
The output format was A5 pages, to be arranged into two
single-sided A4 print jobs performed in sequence to produce
double-sided A4 pages that were cut in half to produce the
double-sided A5 pages ready for perfect binding. I did all the
printing and binding myself, so this was designed to suit my own
production process.

Do you have a guillotine paper cutter, or were you using something else?
Having once used a proper guillotine (clamps paper, then cuts straight
down) I've become very not a fan of the non-clamp swing arm style.

Faced with doing something like that myself, I'd probably be more
inclined to just buy pre-cut reams in the smaller size and print on
those in regular duplex. It would also make the imposition easier.
Although I usually use US Letter, my printer supports down to A6
from the paper tray and I know how to buy "exotic" sizes.

My preference was to have the page numbers on the outside edge of
the pages, so they needed to alternate sides on each page, which
caused me a lot more trouble than it should have.

This is a good style, but not one I'd insist on. Having the margin
wider on the spine side, so text isn't lost in the crack is more
important to me.

Generation of the A5 document was done in Libre Office using a
Libre Office Basic script, which was a choice I soon regretted
because a lot of things didn't seem to work properly. That produced
the A5 document in Postscript, which I reformatted into the two A4
Postscript print jobs using psutils.

Creating of some intermediate format via scripting has been suggested to
me, but no one has pointed me to an easy way to do that. I don't want to
try my hand a *roff template; I haven't done anything but man pages in
20+ years. I don't know Tex/Latex well enough to create my own
templates. And I looked briefly at Sile[*] which tries to modernize
layout and takes Knuth's basic line breaking / filling system and using
that for both lines and larger blocks: the better to avoid orphaned
words or lines. Sile gave me a lot of compile trouble on my (then) older
Ubuntu and I never really went back.

I only ever attempted novels in plain text as input, never any
images.

Many novels have images, too. I selected the (commercially printed)
version of _Moby Dick_ I'm reading now because of the Rockwell Kent
illustrations. Those are not in public domain yet, being from 1930.

I was thinking of books like _Les Liaisons dangereuses_ as probably good
to illustrate. There are a lot of now public domain pictures for that
book, and not so many print versions with those pictures. When I read
it, the copy had no illustrations. The illustrations are part of what
gave it its naughty reputation. The text never really goes further than
"she surrendered all to me" style details.
[*] Search sile-typesetter on Github. I know the author via Usenet.

Elijah
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having already read _Liaisons_ would probably not actually print that one