I found using Librera's Record TTS feature with aitts wavenet plugin you can make decent sounding audio books
Aitts plugin apk: https://web.archive.org/web/20230419204608/https://www.milmike.com/aitts-google-wavenet-voices-for-android-tts
I also use Smart AudioBook Player alongside Librera to listen to the generated audiobook (since librera ends up generating 1-2 min mp3 files). Smart audio book allows clustering of these mp3 files (merging total listening time).