Throughout the first four books, young Gregor has fallen into a world beneath New York city populated by humans, mice, rats and bats, then he found his lost father, nearly lost his baby sister and mother, and discovered that he himself is The Warrior featured in a prophecy surrounding the war between his new friends and the rats. A prophecy which states that The Warrior must die for the war to end.
Not too much to put on the shoulders of a 11 year old boy, but Gregor is a warrior. A rager, to be exact, which means he has an intensity of fighting that, once controlled, makes him the ultimate weapon. And in this final book, that weapon is put to the test.
Now, dealing with his feelings about the war, Luxa, Ripred, and the loss of many friends, Gregor continues his struggle to save Regalia from the Bane, even though he knows to do so means he will not come out alive (at least, according to the Prophecy of Time).
I am a huge Suzanne Collins fan and in The Code of Claw (May, 2007) she does a wonderful job of concluding the series in a touching, inspiring and optimistic way -- despite the violent last lap this installment takes.
Parents should note that these books are quite violent and action-packed, making them very appealing to young boys. This last one was the darkest of all five. But as the end is near, it becomes apparent that, for Gregor, it is darkest before the dawn.
I recommend this series with 4 bookmarks, but IMO it's more appropriate for the top end of the middle-grade set (closer to 12 than to 9).