Oh, did I say 40 books in? My mistake. Clearly the best recap of them all is that of book 54.
Rachel, on the Blade Ship, about to die,Cassie couldn't let Jake make that decision, and Tobias couldn't let me, and those two, by loving us, would have screwed everything up.
It was a war, after all. A war we had to win.
We hadn't asked the Yeerks to come to Earth. They made that call on their own. They're a parasitic species, not very big or impressive to look at, just these snail-like things that can enter your head through your ear. They have a capacity to anesthetize the inner ear enough to allow them to burrow through the soft tissue. It still hurts but not as much as it should.
Or perhaps Applegate just couldn't manage to write what Rachel does next without that structure to lead into it. (Familiarity is comforting, after all, and we all needed some comfort.)