This mirrors the sort of work you've already been doing: extract text, synthesise speech, check storage, then convert the finished WAV to MP3. The exact Piper commands depend on the voice and installation.
Try these commands
df -h /mnt/DATA
mkdir -p /mnt/DATA/Audiobook
ls -lh /mnt/DATA/Audiobook
Tip: Start by reading the command. Then predict what it will do before pressing Enter.
Safety: Never paste a command you do not understand when it contains
sudo, rm, disk/partition commands, or redirects to important files.