Mastering

learning … the mastering process for my recorded book

Have you already recorded it? The whole thing?

There is a new ACX Check analysis tool available.

If the one you used looks like this …

… then you have the old one. The new one has a much shorter info panel, works faster, and on longer performances.

Seems though that using Noise Reduction is necessary

Not always. If you can pass noise by at least -65dB or quieter, then you can usually publish with no further work.

Both the forum and ACX allow you to post a short test before you start pushing tons of sound files around.

Record a ten second sample and post it here.

https://www.kozco.com/tech/audacity/TestClip/Record_A_Clip.html

Do not correct or process anything. Raw voice as you normally perform, cut it to length, export and post it.

Koz