Sorry, I think I was looking at a different thread when I posted that. Not sure how I got my wires crossed.

I think I'm beginning to like strawberry more than chocolate.

I am just letting you know from another angle.  I am exporting from Studio One.  But also performing very final mastering edits in Adobe Audition.  I feel that the editing programs seem to have the final say on reference levels leaving it.
 
I have tested levels from Wave files leaving my DAW compared to CD burning. I am seeing no level changes at all.  My CD burn produces exact reference levels when I burn them.  I am using an earlier version of Nero on my XP machine and CD Burner XP on my Win 7 machine and it is the same.
 
So no level changes for me, if I master a CD to -10 dB rms average then the CD plays back at the same level. (Note: I have calibrated the output from my CD player to match levels that are on my digital mixer.  If anything the CD is 2 dB higher on my system and I have to drop it down 2 db.  But no clipping or distortion anywhere.)  If I limit Xenon to -0.5 dB FS in mastering, that is exactly how the CD is burned and if it were ripped back to a wave file again.  I have checked.
 
It may come down to how well you are pumping up your rms levels during mastering.  I use PSP Xenon as my limiter and it has no problems reaching levels as high as -6 to -8 dB rms, but this represents a very loud master.  I find levels between -10 and -12 mastered rms seem to match a lot of commercial levels in my car for example.  Although some commercial CD's can be insanely loud, but then they sound horrible.
 
CD burns should at least should match your levels leaving your DAW/Interface. Not dropping them down.
 
 

Thanks Jeff, I'll look into this. It seems the simplest of tasks but it's driving me crazy. I'm running the master buss output at -.5 db for peaks but the rms levels are much lower. This is primarily spoken word done by a singer with a big voice. He talks like he's singing and the levels are bouncing from -40 to -6. 
 
 Time for more experiments. I obviously haven't learned how to use the tools.

Try loading your exported file into a new project, and then import the wave file from the CD and compare the two. They should match in amplitude. If not, it's unlikely to be Nero's fault, assuming you're not doing any modifications in the burner (by default, Nero should burn it as-is without modification).
 
If they do match up, take a look at how you're auditioning your CDs. If you're playing them through Windows Media Player or similar, it's possible you're not comparing apples to apples. That's especially true if you're listening to them in your car.
 
It could still be a mastering issue either way, meaning a translation issue. That's a whole 'nother can o' worms.

I've used Nero for years. It's always produced exactly what expected  from my exported wavs.
I understand the opening post so that the exported wavs are fine (?), so I have no idea what could cause
degradation of quality or changes in levels during CD-burn.

For an audio book I would be aiming for a very even voice level in the final master.  For the reasons you have  already mentioned. Playback environments will vary a lot.  And you want to hear the voice clearly all the time.  The way to this is to edit the initial recording.
 
I would record the wide range of input sounds/levels.  Capture all that without clipping.  eg  (use K System level of -20 as the reference)  Maybe even tame the dynamic range going into the DAW a little by acting only on loud peaks first e.g. limiting with high threshold setting.
 
Then I go in and cut the vocal read into smaller segments.  Studio One makes changing clip gain like this on individual audio events easy and possible.  Also as the waveform height does change as you change gain, this end up being a really handy feature for roughly evening up vocal phrases by eye.  Looking at the rms or meat of the sound.  Louder phrases down a little and softer ones up.  You don't even everything up either.  That sounds unnatural.  Preserve some of the initial dynamic range read, just reduce it.  A long read takes less time than you think to do this process.
 
Then you put a compressor over the whole track but you are now feeding in a much more consistent rms levels for each phrase.   Add some makeup gain there.  Light compression settings here.  Like 2:1 max ratio and only 2 to 3 dB gain reduction too. 
 
By the time you hit your limiter and add further 3 dB or so of rms level, you will be sounding very loud on your CD.
 
Forget preserving the dynamic range of the initial read.  That is not the point of the exercise.  It is better to create a very clear voice over read.  All the emotion can be put in at various levels,  but the same emotion can still be conveyed when all the vocal phrasing is much more even in level.  The editing extends far beyond the read itself.  I like to add a final touch EQ and even add a very small room reverb at a very low level to just add some depth the to vocal sound too.  This reverb should not even be obvious.  If there is music and effects under,  then that requires balancing too.  (Small mono speaker at low volume works wonders here !!)
 
 

I am not a fan of what they are doing today either and that is called Cinematic mixing for films and TV series that we are watching with our live streaming options.  They are going a little too far in the opposite direction.  Louder voice parts are getting a little louder but softer ones are getting a little too soft for my liking.  The music and effects are blasting in now at quite high volumes.  I am finding it harder now to hear all the dialogue consistently.  Do any agree with me?  
 
I have got a quality audio system attached to my TV and Apple TV etc and it all sounds great.  Too great though at times.  I have to ride the remote volume now and get ready to turn the music and stuff down and crank the softer dialogue up a bit.  I have got a nice Presonus compressor permanently patched in between the input sources and my Hi Fi power amp.  It tames all this level change for the better for sure, but it still has issues as well not being able to catch some of the loudest peaks on some material. The jury is still out as to how best to set that compressor up as well.  I go from limiting to mild compression and back etc..
 
I was trained in the old days to keep the voice overs more even and consistent and always ensure the listeners can hear it at all times.  Of course playback mediums were not as good then for sure and we have got better options now but we are using them all little too much these days in some cinematic mixing approaches now.  
 
I feel there is a balance there, which is nice consistent voice levels with music and effects coming in but reaching healthy volumes without blowing the listeners head off.  Only adding slightly to the overall level. 

I've been on this project since January. My number 1 consideration was to get good, clear audio with no clipping. I I've managed to  do this. Initially tried to get the RMS levels up but it was always a failure due to clipping. 
This guy has a huge voice and like many singers he tends to build up air and then let loose. This may be great when singing, but when reading, every sentence starts out with a very high level that drops off as the sentence ends. I've explained the issue and shown him the waveforms but it's just the way he rolls. He's also doing some animated voicing and using mic proximity as an effect. It's much harder than any music I've recorded. When you are a rookie recording a guy who has 15 albums under his belt it's difficult to act as engineer and producer.
 
I thank you all for the continued feedback and you patience. Don't bail on me, I'll figure it out.

As for cinematic mixing, I'm with Jeff. In fact, I've stopped going to most movies because I come out feeling like I've been assaulted. There's no way you should come out of the movies with ringing ears, but that's what happens. It must be in deference to the "ear bud generation". They've scuppered their hearing and in order to excite them movies are equipped with weapons grade audio. My Sony TV has a steady sound option. I used to use it but the internal compressor is messed up and it won't release at times. Loud passages trigger it but then you never hear the quiet passages because the release time is way too long.
 
1st world problems to be sure.