Is there a difference between Trinary, Ternary (in operator specific context).

No. Ternary is the more common term for the ? : operator, though.

Why is Trinary used primarily in the Camel.

I suspect that one of the writers of the Camel book was engaging in some word-play, or was taking a 'dig' at someone/something, or it was an expression of a pet peeve. Maybe merlyn will see this node and can shed some light on this.

How could two very knowledgeable people not know of such a term.

People with expansive vocabularies can still not know of the word 'trinary'. Just because a person doesn't know it doesn't mean that it is not a 'real' word, though. An entomologist may not know the etymology of 'entomologist'. You can google it or check dictionary.com. Or one of those dead-tree word-looker-upper thingies at the library or bookstore. ;)

Possibly some background history on the terms themselves.

The two words are pretty much synonymous, though 'ternary' enjoys more popular use. Both words have Latin roots... 'tri-' and 'tern-' both mean 'three'. Blame it on those darned Romans.