We don’t recommend any videos. The ones we’ve seen are either perfectly correct—for the wrong Audacity—or have errors.
That’s not to say you can’t pick up good hints and tricks. I particularly like the video of the guy recording in his bathroom, and it sounds like it, and he magically converts the voice sound into a studio by throwing a blanket over his head. There are actually hardware home-brew and commercial versions of that blanket.
I don’t know if this appears anywhere, but once you start a project, Do Not Update Anything. The forum is full of people who automatically updated their software at each opportunity and very nasty things happened.
Audacity isn’t a massive corporation with offices of people doing quality control and version testing. We do as well as we can, but problems slip through. Audacity is available on all three computer platforms and multi-languages, so testing is monstrously difficult. There was one recent stability problem that only appeared in one Asian language.
Which version of Audacity do you have? Don’t do any more work until we have a little more info.
Audacity Projects can have stability problems and putting one audiobook in one monster Project is dangerous. This goes hand-in-hand with the Audacity version.
It is recommended that you work chapter at a time rather than continuously updating a single Project. You have to submit to ACX in discrete chapters anyway.
We publish Audiobook Mastering which will turn almost anything into a performance meeting ACX RMS (loudness) and Peak specifications. If you recorded well in a quiet room with no echoes, noise should fall in and that may be all you need.
That’s the long version of these three tools.
We also publish ACX-Check which will tell you if your performance meets all three requirements.
It reports in a nice, brief panel.
You are almost certainly using Noise Reduction wrong. It doesn’t reduce noise to dead zero and it’s not supposed to affect the sound quality. Reduction Recommendations depend on the results from ACX-Check. All these tools hold hands with each other.
I’m not sure where to go with your single, massive Project. Do you let your Mac do Time Machine backups, or do you do any kind of backups manually?
Let’s start with Audacity version. Audacity (upper left) > About.