I've just started preaching through Ecclesiastes. You can find the introductory sermon
here. I argue that the book is not negative, but realistic. Romans 8:20 is a key verse (the word
futility is the same for
vanity in the LXX) - and so we should consider the resurrection as central in understanding the book. One other consideration is that the book emphasizes to us the wisdom of Christ, though perhaps not as much His work - but this is no problem because the Law is good.