While the ending is often criticized from a technical/writing standpoint, and reviled from a character/fandom standpoint, as far as what it was saying, it was masterful.

Assuming you know about the character death(s), I have to say that the narration by a dying character was among the most heartbreaking things I have ever read in fiction, and came the closest to choking me up since the final scene of Braveheart. And it was early in the book so as to pretty much undercut any sense of triumph from the subseqent scenes of wrapping up the ultimate win. The bad guys are surrendering, the villains are being taken prisoner, the dubious allies getting their comeuppance and rebukes, the final victory being pulled off, at long last, and all the while that six chapter death scene is hanging over your head, casting a pall over the whole thing. In fact, the war is basically won by the end of penultimate book, with just one thing left to do to clinch the victory. The finale is an examination of the costs of the measures taken and the aftermath of the fighting.

In many other works where you have characters still let down or underwhelmed by their victory, and you feel kind of impatient with them, and wishing they'd get over it. In this, you completely understood and kind of resented the happy lives the other characters moved on to. The ones who didn't recover from the war didn't come across as party-poopers, instead you wondered how the ones who moved on could do so so easily, especially after all the lost characters should have meant.

But Applegate, for all the ostensible kid stuff and fun and zoology lessons, was really writing about the effects and consequences of war. And her message stuck the way so many more serious or mature works on the same theme failed to do so. What is more, as a history buff, I can recognize the same thing happening in other wars. World War Two was started over the freedom of Poland, and ended with Poland being consigned to almost fifty years of tyranny, and the aggrandizement and increase in power of one of the two nations that attacked and partitioned Poland. The more you look at that war, the more the Poles and Finns look like the real heroes, and they were the ones who got shafted. And that's what happened with the Animorphs. I can't think that's a coincidence, given the historical awareness displayed throughout the series (which was extremely gratifying from a history buff's perspective). The most heroic characters get shafted, and in fact, the endings and fates and (attitudes about the same) of each character is in inverse order of what they deserved.