Just to be clear - עצוב can be used, then, just as 'sad' in English, to describe both a felt emotion and something that causes it?
So, from the discussion on "Bad dog!", it follows that the only reason this is "A sad book" instead of "a book is sad" is because the latter is illogical?
I guess I was getting mixed up with pronouns--which don't require the copula, right? Are they the only thing that doesn't? For example in an "x is y" sentence? Then you have "the x is y" and "the y x," where x is the subject and y is the adjective... right? Guess I need to study more :)
You need a copula when equating a noun to another noun, ספר הוא חבר. You also need it to express that a noun is an adjective, but only when the noun is indeffinite - הספר עצוב would be a whole sentece, the book is sad, no copula necessary, but ספר עצוב is just a noun phrase, sad book.
Is this correct? To say 'the sad book' = הספר העצוב, and to say 'the book is sad' you'd say הספר עצוב?