CCEL, the Christian Classics Ethereal Library, (which happens to be my current employer), has hundreds of books in their ThML format. (Their library is free to use for non-commercial purposes).
I've recently written a script to convert that into gemtext, which you can see here: https://git.sr.ht/~earboxer/thml2gmi
I've cobbled some scripts together for browsing their library on gemini
gemini://gemini.zachdecook.com/cgi-bin/ccelindex.sh -- currently, the script fails on some of these books.
The ThML format is XML, and provides for different sections, has footnotes, (optional) page break indicators, scripture and author references/links, and has good header information as well, which might be of interest in organizing a library. (More info can be read at https://ccel.org/ThML/index.html).
I would be interested in converting some of these to whatever format booksin.space desires for the library, and/or providing scripts to render this as gemtext.
I'd never heard of ThML before. If you have particular texts you'd like
to be included in the booksin.space library, I think converting them to
gemtext format and filling some basic metadata into a YAML template is
all it would take to contribute them. (Things have stalled out around
here so this template does not yet exist!) I'll try and write up a
template this evening or the next so people can start adding books.
Thanks for your interest :)