Most favorite parts were when Rachel did the elephant morph in her room, when she had to see Chapman in his office and the end where Cassie evades that Controller.
The Yeerks' plan was sound. They further attempt something similar in The Proposal, with the Roger Tenant geezer. Jeremy Jason McCole was shown as a real self-centered ignorant dumb jerk… Ready to sell out his own species. Even when not infested, he cheered on how cool the Visser was, having witnessed his first extra-terrestrial. What does that say about the human?
Also worth noting here, that in this book, the Visser had first acquired the Lebtin Javelin Morph. I am uncertain right now if this animal (or another of the Visser’s aquatic creatures) is the one that is meant to be from one of the Andalite Moons. But the implications of the Visser acquiring an animal during this stage of the war, would support the plot started on book 8: The Alien and further more the traitors found on 18: The Decision.
I think this kind of thing happened to Rachel, because reckless of the bunch. I am not sure how the other Animorphs would have reacted in her shoes. We know Jake and Marco wouldn't have been much different, Cassie would have certainly have used some sensibility, Tobias and Ax may well have pulled out for all I know.
I assume because everybody has some sort of an allergy to something, it was natural and fitting how a body can have an allergic reaction to such thing as a morph.
Beyond the allergy thing, though, I found there’s also another spin given to the morphing tech shown in this book. Where upon doing a Rachel-morph, Cassie states that her brain keeps wanting to do stupid things. Doesn't this suggest that the DNA of a sentient being carries their personalities and such?! Such a statement is debatable, and it would be believable in the Animorphs Universe, if it had been carried out throughout the series. As it is, we only get this comment on the topic here.
I agree with TheSoulEater quote comment above. A 'partnership' suggests a reciprocal benefit or exchange. In technical terms, the Yeerk has absolute control. Sure the host gets slips of the Yeerk's emotions, memories and experiences, but it's definitely not even.
A host is to a Yeerk like a car is to a driver. The car is susceptible to whatever the driver's motives, experience and forces. Whether the car is a more powerful thing than the driver on its own, one thing is undeniable, it's going where the driver wants it to go.
Such is the relationship within ANY TYPE OF CONTROLLER, in my view.