I did not experience Animorphs until the turn of my thirties (as my spouse read it out loud to me book by book on road trips) and it was still GRIPPING.
It is about children unwillingly enlisted into an intergalactic war against a race of brain parasites, completely alone with one alien friend on the planet earth while they await the arrival of reinforcements from several galaxies away.
Able to collect the DNA of other species as new forms they can take at any time, the kids must engage in complex, harrowing guerilla (lol gorilla) tactics to escape slavery or death and protect their families, taking advantage of the unique biology of different creatures to pull off incredibly creative sabotage and espionage missions against people taken over by the slug-like Yeerks.
The books explore the horror of war, torture, bigotry, slavery, genocide, genocide and more genocide with a heavy overarching theme of whether the struggle to survive is “worth it” in the face of inevitable extinction (nothing can last forever) or all the cruelty and tragedy that has to happen for a civilization to march on.
Animorphs books can get more emotionally tense than an adult horror novel and sometimes just as gory. You always hear “lol I can’t believe they sold this to kids!” about all sorts of things but Animorphs may as well be THE “how the fuck did they get away with this” series.