Not only should the book of
Job not have been left out of the
Old Testament, it is probably one of the most philosophically important books in it, especially from the point a view of a
modern reader.
In many places in the OT the people of Israel recieve the injunction to "think of the torah day and night" - in other words, to ponder and study the religious and historical writings continually. However, Job is the only book which introduces intellectual technique to this way of life - without Job and his friends providing an example of rationalising and hypothesising about God, the practice of studying the Torah, a cornerstone and, some would say, the be all and end all of Jewish orthodox life, would be nothing more than empty parotting.
I am inclined to agree with lemuru that the "happy ending" suffixed to the book is probably a late embellishment, for Job's real reward is a one-on-one with God - a privilege reserved only for Abraham and Moses elsewhere in the OT (I could be mistaken here, but i'm pretty sure at least that the prophets, the kings and the other people to have received revelations did that in dreams or through angels-messangers).
An interesting take on the Job story is the belief (by no means uncontroversial, but still widely spread) that Job's legendary righteousness doesn't pertain to his life before the disasters strike him, but to his life after them - and that his doubts, his questions and his courage to demand answers are a part of that righteousness. Job is the first Jewish (well, we're not told he's Jewish, actually, but morally Jewish) philosopher. He doesn't take anything on anybody's authority, and remains steadfast in what his friends consider prideful and defiant insistance on his own innocence. This is what he is rewarded for, not any kind of blind adherence to God's word.
This interpretation of the book of Job and its place in the OT is one of the things that endears the Jewish religion to me (as much as a religion could ever be dear to me) - the fact that from the very primeval outset it was a philosophy which encouraged people to think and to act for themselves, and not just to follow blindly in the footsteps of others.