Did I hear someone whisper “Book SPLOT”?!
There are a few examples of this sort of publishing pattern at academic samizdat - IndieWeb and book - IndieWeb that use a variety of technologies.
The easiest method is obviously to publish a .pdf copy and simply link it. If you have a text version of it and want .epub or .mobi files for e-readers I’ve got infrastructure for converting text into those I could put into service for you. I’ve done it in the past for Stommel and Morris’s Urgency of Teachers. (I have a small publishing house on the side and can help you out with ISBN numbers for much cheaper than usual if you like.)
@kfitz has done it a few times during the process of writing and subsequently publishing books, so she may have ideas/opinions. If I’m not mistaken, she used CommentPress, so that may be most comfortable for you from within the WordPress world.
Jeremy Keith has an awesome example at https://resilientwebdesign.com/ and if you pinged him, he may have a flat html file “shell” that you could cut and paste into. (Or you could view source and manually get the same result.)
Amy Guy’s example on Github which she published using Github Pages is nice and could make a fun little project for you as well.
It would be so much nicer if there were a one click install of PressBooks, but I’ve quit holding my breath on that front. (Maybe it’s a future possibility for Reclaim Cloud though???)