Hmmm. I'm sort of confused by your question, but I'll give it a shot.
The Conan of the Isles book on my shelf isn't by "Roberts" and perhaps you meant "Robert E. Howard"? In any case, it's book #12 in the Ace Books Conan series and my book is written by L.Sprague DeCamp and Lin Carter. Howard actually had no part in its creation.
So ... assuming we are talking about the same book, I'll give you an answer. As you said, it will contain spoilers so fair warning to those who might not want to know.
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Conan has learned that by sailing far to the west he can find a new continent full of other barbarians who have escaped the doom of Atlantis. These apparently are the Mayans, who will name his ship Quentzalcoatl when they see it someday. The book ends with Conan sailing off into the Atlantic ocean and toward these new lands.
We can assume he (or at least his ship) makes it to South America because of the passage about the Mayans, but the series ends and we are never told what happens to Conan.
Try to keep in mind that none of this happened, at least as far as Robert E. Howard is concerned. Howard's Conan does become King of Aquilonia but never goes on these "last adventures" written by DeCamp and Carter.
