Join-merge-consolidate many audio book CDs into one or a few albums in iTunes 12 / PC?
High-Level Steps to Merge Multiple Audio Book CDs


Read the "Meta Data Tips" below (it could save you some time).  
Load CD 1.  
Sort the tracks into the right order. [ Click Options > Get Track Names ]  
Join the tracks. [ Click Edit > Select All and then Options > Join CD Tracks ]  
Fill in the initial meta data (Author, Title, Spoken &amp; Audio, etc.). [ Click CD Info ]   
Import the CD [ Import CD > AAC Encoder + Spoken Podcast + Use Error Correction ].   
Wait for import to finish. 
Go to View Music and find the imported CD (to assure yourself it is there).  
Repeat Steps 1-7 until all CDs have been imported.  
Change each CD from "Music" to "Audiobook" and set the playback and shuffle properties.
[ Right-click > Get Info > Options > media kind = Audiobook ] 

[ Check-in Remember playback position and Skip when shuffling ] + [Click OK].



Notes:
(a) The CD will disappear from your Music library and reappear in your Audiobooks.
(b) Don't be alarmed if iTunes moves all or a subset of your books at once instead of one-at-a-time. Sometimes it happens; sometimes not. I haven't figured out why. I am, however, grateful when multiples are (correctly) moved all at once. 


Find the CDs in your Audio Books. [ View > Audiobooks ]  
For each CD that you want to include in a particular merge, retitle (rename) that CD so that it has exactly the same title as all other CDs in that merge. 

[Right-click > Get Info > change the Title ]. 


Example: My book has 5 CDs (Superbook Disc 1, Superbook Disc 2, etc. I want them all in one merge. So I change all Titles to Superbook-5CDs. As I change each title, that CD is merged with the other CDs of the same title.


To see what you've merged at any time, just click on the new CD title. You should see an expanded list of the CDs contained in that merge.


Meta Data Tips


You can get to meta data fields by clicking on the "CD Info" icon or by right-clicking and selecting "Get Info." And there may be other ways. Do it one way and you might see a subset of the fields available to you via the other way.
When asked "Are you sure want to edit multiple items?" then, yes, of course you do!
Be sure the Disc N of N data is correct.
While working with your first CD, take notes (or a screenshot) of your Author, Composer, etc. meta data so that you can repeat it exactly for each subsequent CD.
Keep your meta data simple. Be sure to fill in the fields that will help you when searching and sorting in iTunes. And to fill in those fields exactly alike.


You need to be careful about your meta data entries because iTunes is very picky about matching and sorting. Any differences (spelling, spacing, capitalization, punctuation) will likely cause you difficulty in subsequent steps.

For example, mis-matched meta data could cause your imported (and as yet non-merged) CDs to be scattered instead of appearing next to / near each other in different viewing modes.

Credits

I pulled the above together over several years (from iTunes 8 until now with iTunes 12). For those of you with older versions of iTunes, here are the sites where I found helpful information:


www.aldoblog.com (as of 2/24/15 his Audiobook postings are returning 404 errors, though they were working last week. Maybe the site owner is updating the content?)
http://oakroadsystems.com/genl/itunsrip.htm
http://criticalcreig.org/2013/06/how-to-import-cd-audiobooks-into-itunes-11/
http://www.mcelhearn.com/
(And yes, even on ehow) http://www.ehow.com/how_5885990_merge-audiobook-discs-itunes.html