If you're interested in further text to speech missions, I just got Piper (open-source text-to-speech engine) running happily in a Docker container on my Mac. Effectively "free", high quality, fast-generating text-to-speech.
Piper is great, but nowhere near the quality of the OpenAI TTS voices.
I'd find Piper too jarring for audio books because of the big quality difference, but I actually prefer it for things like AI assistants as I don't necessarily "want" AI assistants to sound perfectly human, and prefer the stylistic choice of having them sound more computer-generated.
Check out their voice samples: https://rhasspy.github.io/piper-samples/ (or make your own).
Happy to help you set it up locally...
https://github.com/rhasspy/piper