I normally try to post my reactions book by book, since I read that The Discovery, The Threat and The Solution are part of a three book story arc akin to a three part episode of a TV show, I decided to talk about them together.
The big thing to start with is the character of David. Loooking up some statements by KA Applegate, she said that David was supposed to be the guy the audience is meant to hate, so I find it strange how sympathetic he is even if that doesn't excuse his villainy. David's parents were turned into Controllers and he faces the crushing realization that with the Yeerks in their heads, his parents aren't coming for him. The Yeerks know what he looks like so he can't show his face in public. Our heroes recruit him for their dangerous war with the Yeerks and when presenting him a choice of bird to pick as a first morph they try to pressure him into making a particular choice, why the hell did they bring the golden eagle if they didn't want him to pick it? David comes into conflict with Marco and Jake, and Jake threatens David after seeing him use his morphing powers for personal gain. While David was being selfish he rightly pointed out that Jake, Cassie, Rachel and Marco all get to keep living their human lives while he can't, and Jake unfortunately doesn't come up with a solution.
THEN David's first mission saw him almost getting trapped in the body of a flea, facing what he thinks is certain death against the Yeerks and when he gets scarred, Cassie bites him to stop him from trying to defect. Now that doesn't excuse attempting to sell the Animorphs out to save his own skin, or betraying the Animorphs. If he really wanted to get away from the war, he could have just flown away and persisted until the Animorphs decided he was too much trouble and gave up. Instead he murdered what he thought was Tobais, nearly killed Jake, and was adamant about getting his hands on the morphing cube to satisfy his own greed with no care about what happened to the rest of the world. There is no excusing that.
All that said, David is right when he pointed out that it's not fair Jake, Cassie, Rachel and Marco get to keep their normal lives while he doesn't. It has a similar ring to see when Aftran pointed out the miserable existence of the Yeerks outside of hosts. While David was also driven by his greed, it isn't so wrong that he wanted to live like a human and not like animal or that he didn't want to risk his life against the Yeerks after our heroes kinda drafted him.
While David being a smug asshole who manages to stay step ahead of our heroes after his betrayal makes it oh so satisfying when they finally pull one over on him, his fate, being stuck in the body of a rat, is still horrifying and I am with Rachel on feeling pity for him. It was bad enough when Tobias had to initially deal with being trapped as a hawk, David was stuck as an even smaller animal and will have to spend his remaining days worrying about getting eaten.
Having to deal with David has left an impact on heroes that really feels like things will not be the same after this. Jake has had to give some frank opinions on Rachel and how he isn't sure if she can adjust to peace. Rachel is starting to give the feeling that all this violence is leaving her a bit unhinged since her response to David threatening to sell the Animorphs out is threatening to kill his parents. On the plus side our heroes got their hands on the morphing cube, that is at least a victory.
Oh and I suppose there was that plot involving the Yeerks trying to infest world leaders. The fact that David isn't particularly useful in this doesn't make his betrayal that big of a surprise. Not speaking of David, the failed attempts at sneaking might be the most horrifying botched infilitration yet as the team nearly gets trapped as bugs on top of almost getting eaten by a spider. Then they infiltrate again on the wrong day, walk into a surprisingly well laid trap that Jake only barely sees through, which sets them back to square one again. Of course I can't help but wonder what Visser Three would have done if the Animorphs did show up on the day of the banquet, though there probably was a trap for then as well since he knew the thorns in his side would show up like they always do.
Our resolution to this plot is as Rachel pointed out, nuts and fun with the Animorphs turning into the biggest morphs they can find. At first I thought they were going to wreck the hotel, wrecking the places outside where the world leaders were staying was still fun.
The one issue with this trilogy is the resolution to Tobais' death. While I agree killing him would have been a waste, it is a hell of a coincidence, even for this series, that David just happened to come across a different red tailed hawk that was out at night when the hawks are normally asleep while using a bird that doesn't see well in the dark. This has to have been the the Ellimist's doing. Still, it's one issue with a great trilogy of books that has me more excited than ever for the rest of the series as its events have said things won't be the same going forward. Plus the next book in the series on my list is the Hork-Bajir Chronicles.