I was under the impression that Jones was returning the Unicorn's horn in "Interior World", because he got rid of it in "Unicorn's Legacy". I just picked up the McCoy novels on E-bay, so I read the other's over that I hadn't read sice they first came out. What are we talking? 10 years? Anyway, maybe the Phil. Stone was in "Interior World". It seems to me the authors are dealing with too many of the same things. "Interior World/Hollow Earth", "Phil's Stone/ Iron Phoenix" There are other mystical archaeological finds still to be discovered.
Oh, and by the way, I found the Cliff's Notes of Jones-
http://www.innermind.com/youngindy/info/indy.htm
It makes me feel like I'm in the know.