Bien vs bon(ne)
I've run across a couple of sentences in the 'adjectives 2' lesson that are making me question when to use "bon" and when to use "bien". I had understood that the difference is essentially the same as "good" vs "well", i.e., adjective vs adverb. But I've gotten some sentences that are making me wonder if there's more to it. (unfortunately my current browser is preventing me from accessing the discussion pages for those sentences from the lesson).
"C'est bien" is given instead of "C'est bon" and "Sa lettre est bien". instead of "Sa lettre est bon"
Wouldn't this latter mean essentially that the letter is doing a good job of existing, rather than that the letter is good? In the same lesson, I got the example, "Ses livres sont bons" and I don't see a difference between this construction and "Sa letter est bien" that would explain why one is bien and one is bon.
Any suggestions? Are these maybe idiomatic expressions that are more naturally used even if not technically grammatical? (like saying "it's me" instead of "it's I".)