Working with Audiobooks
I've dealt with couple of audiobooks in the past and would like to share my experience with you, along with its technical process.
(I'm assuming that you've the recorded files and the script ready to handover to your editor.)
This process is divided into 4 parts -
Cleaning,
Editing,
Mixing,
Mastering.
Now let's dive into each process.

CLEANING - It means removing the
background noises (dirt),
electronic noises
Mouth clicks/chaps/licks,
small breaths,
plosives,
windy rustle,
clipping/distortion,
Gain Staging.
We can do more during the cleaning process, like controlling the Esses (sibilance), applying EQ, etc. But try to avoid doing this, because we will have much control during the Mixing Process.
2. EDITING - Import the files in your editing software. I use 2 screens while editing an audiobook. One screen for editing and one for reading the script simultaneously. The main goal of editing is to make the audio sound seamless without any unnatural cuts.
Sometimes the speaker stretches the words, mispronounces, stutter without even realizing, so you might have to find replacement words in those audios to replace these mistake. Or try a combination of parts of various words to make one word.

This process mainly includes removing of -
Filler Words
Retakes
Stutters
Coughs, Sneezes, Burps
Umms and Ahhs
Breaths
Unwanted noises e.g. Dog Bark, Door Knock etc.,
Gain Staging.
This process requires carefully listening and focus of finding errors, sitting for long hours watching the given script and making the best version of the audio.
3. MIXING - This is where the audio comes to life. Clarity is added to the audio and annoying frequencies are carefully controlled. There are hundreds of plugin, and couple of software which can do this job amazingly. But it can get overwhelming for beginners while mixing their audio. There are typically a few process which can enhance the audio -

Equalizer (EQ)
Gate
Compression
De-esser
Gain Staging
It requires a good amount of experience and expertise to use these plugins flawlessly. Each audio type is different and each speaker is also different. To use what and when, and the requirements of your clients totally affect your plugin choices. Even the plugins can be same but their order can be different.

4. MASTERING - This process decides whether your audio will be accepted on the streaming platform or not. Each platform has different requirements. These are the requirements for audible - ACX Requirements
In a nutshell, this process controls the loudness of your audio and its peaks, so it doesn't sound too loud or too quiet. It also controls sudden jump of audio and trims them down.


Use an audio meter of your choice to check the levels of your audio and check if its meeting the requirements of your streaming platform. If not, then make adjustments in the Limiter Plugin of your choice.
This whole process can be simple and complex at the same time. That also depends on the client and his working process. At the end of the day, you have to make things easier for them because that's what you're being paid for.

Do comment if you have anything to add.
Thanks,
Aman Dembla