Thank you for the release! I've used ECT (https://github.com/fhanau/Efficient-Compression-Tool) to losslessly optimize the PNG files to reduce the book's size by 14.6% with settings "-9 --reuse". It's not a huge difference, but you might want to think about optimizing your files this way. Here is the download: https://files.catbox.moe/un2arx.zip
I also used cjxl (https://encode.su/threads/3564-JXL-version-0-3-released?p=69082&viewfull=1#post69082) to losslessly convert to the new format, JPEG XL, for a whopping 49.5% savings! Settings used: -q 100 -s 9 -E 3 -I 1 https://files.catbox.moe/htox5o.zip