My dad once said to me, "You give a lot of books three stars." I do.
This review contains spoilers -ish
Let's judge the David trilogy by the Star Wars Test.
The Discovery doesn't quite pass because there's nothing resolved at the end. I suppose you could say the initial bit of the blue box story is wrapped up, but the book itself ends on a cliffhanger in the middle of a mission.
The Threat passes. The stakes are upped. The HOS mission is still unresolved. David's evolving as, well, the threat. The Animorphs are left in a very dire situation at the end.
The Solution also passes in that it's just a great summation of the main plot line. There could be a little more to the HOS mission, but I'll sacrifice that for the gratification of seeing David's story line wrapped up.
As a kid I would just reread the end because of how satisfying the whole plan is. It's definitely creepier to me as an adult, but it's still clever.