J. Stafford Wright on Ecclesiastes' place in the canon:

[Ecclesiastes] is a unique book, and its omission from the Bible would be a definite loss. Quite obviously it is not the last word on the problems of life, for it belongs to the Old Testament and not to the New. But its solution is along the consistent Bible lines that appear in both the Old and the New Testaments. Is it only by chance that Paul in Romans 8, after speaking of the vanity of the whole creation, goes on to speak of the sufferings that create a problem even for the Christian, and [of] the confidence of the Christian in his daily life that all things work together for good for him? "All things" means those … events that we share in common with all mankind, where the race is not to the swift nor the battle to the strong. The world is not weighted in our favor. But the same things which break the man of the world can make the Christian, if he takes them from the hand of God. Go on looking for the key that will unify the whole of life. 

You must look for it. God has made you like that, sore travail though it be. But you will not find it in the world; you will not find it in life . . . No philosophy of life can satisfy if it leaves out Christ. Yet even the finest Christian philosophy must own itself baffled. But do not despair. There is a life to be lived day by day. And in the succession of apparently unrelated events God may be served and God may be glorified. And in this daily service of God we may find pleasure, because we are fulfilling the purpose for which God made us. (From "Interpretation of Ecclesiastes")