Here is my two-minute attempt to summarize the philosophical point of this difficult masterpiece:
The book, I think, should be divided in the following way:
Part I: Job and Job's Wife
The point of this story is to give the situation and show the righteousness of Job.
Part II: Job and Job's friends.
This, I think, should be treated as a distinct story. It presupposes Part I, but its point is different It has two parts:
A. The debate
B. Elihu's speech
Part III: Job and the Lord
This isn't quite a distinct story, but it is a distinct part; Elihu's speech links this part of the story with the debate, and it is this part that closes both Part I and Part II and unites them together.
The part that is especially interesting philosophically are Parts II and III.
Job's friends are sophistical reasoners. They take a truth with which everyone in the book agrees (Job 9:2, 12:3, 13:1-2), namely, that God who is just and wise, gives affliction to the wicked as just punishment. They see a fact before their eyes: Job has been afflicted. They fallaciously conclude: Therefore Job is wicked. Job denies this, of course. The three friends, taking this reasoning as solid, they take the original principle, which the book is clear should not be held in doubt, to depend entirely (by modus tollens) on the claim (which we, God, Satan, and Job all know to be false) that Job is wicked. Therefore, to preserve the justice and wisdom of God, they must prove that Job is wicked; Eliphaz goes so far as to list the particular types of sin Job must have committed to be afflicted in the way he has (ch. 22). They cannot, however, prove that Job is wrong in thinking himself righteous, so by their own principles they put God in the wrong (13:4-12, 32:2, 42:7-8). Their fallacy is a plausible one for the author to note: we fall into sophisms like theirs all the time, and all too often talk as if basic principles depended on our (fallible) reasoning, rather than being the basis of that reasoning.
Elihu, I think, calls attention to this reasoning; he is concerned that all the parties in the debate have reached the point where they treat the justice of God as if it were dependent on Job, so he reminds everyone that there is more to this than the debaters are allowing.
The Lord's speech is sometimes portrayed as a display of power to silence Job; but in actual fact, God does not emphasize His power but His wisdom. This is a very different sort of argument, and is much more appopriate to the actual debate. God tells Job: I know what I'm doing; can you really say you have a better idea what's going on than I do? And Job, who has admitted this in theory all along, realizes that he has lost sight of this in practice: he has forgotten how small he is, and how little he knows.
Besides sinning against God by treating His justice and wisdom as though it depended on their (false) conclusions about Job's sins, the three friends also sin against Job (16:2, 19:2-6, 21:3, 21:34, 26:2-4) by letting their sophistical reasoning cloud their compassion and friendship - a trait all too common to the human race, I'm afraid. Thus it is very fitting that God's forgiveness of them is conditional on Job's intercession (42:7-10).
The restoration of Job's life, far from being tacked on as some commentators make it, is key to the story; if it is ignored, we are left in something like despair. It reminds us that the Lord loves His servants, and that the basic principle that has been agreed upon by all the members of the debate (and is thought, incorrectly, by many to be the conclusion against which the book is arguing), that God punishes the wicked and rewards the just, is completely correct. What the book shows, however, is that both clauses of the principle are more complicated than they look. God can be merciful to those who sin (as he is to Job and his friends), and he can let the just be tried; the wicked still are subject to punishment and the just to reward, but we cannot say that at every moment the wicked are punished and the just rewarded, because God is doing far more than that in his interaction with the world; we cannot conclude from someone's being in affliction that they are wicked, nor from their being prosperous that they are just; and, most important of all, we are taught the importance of compassion. The two interlinked themes of the book are justice and compassion. They cannot be conflated or separated in a simplistic fashion, for they are definitely distinct, but they are unified in wisdom, so they cannot be treated apart. God is just, and God is compassionate; we must be just, but we must also be compassionate; in both cases it is wisdom that make possible the unification of the two. This seems to me the ultimate moral of the book.