How do you ensure that the audio quality remains consistent throughout the entire book?

Above all, the space you record in needs to be dependable. You need to have a dedicated space that's treated well to block out exterior noises and control reverb so that you have a nice, clean sound. Keep your microphone in the same position throughout, record at the same volume level, and ensure you are comfortable to record for long stretches of time BEFORE you begin.

DON'T record in a different space, change your equipment or record at different levels. Make no changes in your setup whatsoever. You can only fix so many issues in post, you'll save yourself so much stress by just getting your recordings clean, consistent and controlled.