I've spent some time on the forums researching this topic and while I may have missed it, I haven't found a thread that seems to deal specifically with the question of what are the best reference works about the Hyborian Age itself. I'd like to draw as fine a line as possible between works more focused on REH himself or Conan in particular and identify those dedicated (or at least primarily so) to exploring the history and cultures of this amazingly rich world (with a terrific bonus for any that contain nice maps and illustrations). It very well may be that there is no present work with this specific focus as it appears those that I am familiar with inevitably weave all the above mentioned threads together in one work.
Here is the very short list of works (in no particular order) that I have identified as at least loosely fitting into this category thus far but invite comments from those who actually own them and from others with additions to the overall list:
- Falconer, A Gazeteer of the Hyborian World of Conan
- Rippke, The Hyborian Heresies