Sections 51 and 59 are both PL OK.
As you were asking about how to make better audios, here are a few thoughts for next time around.
As I said before, I don't think I'm in a position to comment on the performance side. What I would say, having asked for similar feedback myself in the past, is that there seem to be two schools of thought as to the ideal audiobook reading. Some people seem to prefer a reading that is fairly flat so that the listener has the opportunity to interpret things as they want while others (I think the majority) seem to see it as more dramatic than that. Some will go so far as doing character voices, but it's hard to do those so that they enhance rather than detract. I'd suggest figuring out which of these ideologies you subscribe to so that you can interpret other comments appropriately.
Where I can help is with taming the mouth clicks, sibilants and boxiness. None of these are excessive, but they're noticeable when listening through studio headphones. The reason I was asking about 'out of the box' Audacity is that these are best dealt with by installing plug-ins. I'll include links to the versions I know of, but be aware that some of these change when the plug-in is updated.
The easy one to deal with is the mouth clicks. We all make these - it's down to bubbles of saliva popping as we talk, but we don't hear them normally as we don't get as close to a mouth as a microphone does. One of the Audacity authors has written a plug-in that has an almost magical effect on mouth clicks and can be downloaded from the first post on this discussion thread: https://forum.audacityteam.org/viewtopic.php?t=79278 It works best applied to sub-section selections where you hear a click - applying it to the whole audio takes a surprisingly long time (several hours!) and can damage legitimate sounds such as "t"s and "g"s.
Sibilants are the high-pitched sounds that sometimes accompany "s" or (more rarely) "th" or "f" sounds. There are general de-esser plugins (indeed, there's one on the same post as the de-clicker) but people tend to hiss at a very consistent frequency so it does less damage to your audio just to cut that out. The tool to do this is a parametric equaliser. I actually paid for one (which I can't access as I put it on the work pooter and I'm working from home during the pandemic) but there's a free one at the bottom of this page: https://2ndsenseaudio.com/plugins/ I couldn't find a good beginners' guide to doing this; the best I could Google was https://music.tutsplus.com/tutorials/how-to-use-a-parametric-equalizer--audio-2301 but if that doesn't make sense then let me know and I'll try to see if I can do a screen recording for you. The sibilants are in the 5000Hz-10000Hz range, though some male voices can drop as low as 3000Hz. Your sibilants aren't excessive, so don't be overenthusiastic in the cuts.
Once you've got used to the parametric equaliser, it'll do other jobs for you. You can use it to cut the boxiness (100-500Hz), roll off the bass (which covers most of your background noise) and add brilliance and clarity by slightly boosting the frequencies above 10000Hz. It's not easy to use if you're new to them, but as you gain experience the sound will become more polished.
There are two other plugins that you may want to be aware of. One is ReplayGain https://forum.audacityteam.org/viewtopic.php?t=63067 - this allows you to check the loudness of audio within Audacity so you're not having to go back and forth between it and the Checker. The other is LoudMax - this is a limiter (a bit like a compressor) which is more transparent to the sound than Audacity's compressor to my ears. I think it adjusts the power more gradually. You can download it from https://loudmax.blogspot.com/ but note that at present Audacity only plays well with 32-bit plugins.
Hope that's of some use - let me know if you want me to unpack any of it. Probably best to do so via private message rather than clog up the book thread though.