What is this about?
The first Animorphs was released in June 1996 and the series didn't finish up until May 2001. Most of the books were ghostwritten, though with 12-14 books released a year, with 64 books in total, it probably would be impossible to do otherwise. Over time, the universe began to develop, and later books featured a variety of alien species and planets, to varying degrees of realism. Yeah, it had the flaws you could expect from a series written by many different authors, but it also had a deeply engaging story and excellent characters.
Animorphs was very, very dark for its target audience. It was obviously written for elementary school kids, but many of the concepts (war, murder, betrayal, enslavement, murder, PTSD, torture, murder) would have been better for young adults. It also didn’t shy away from showing the main characters dealing with the psychological impact of their actions. Despite this, or perhaps because of it, Animorphs was a commercial success. Kids loved this series, and the concerned adult reviewers could go screw themselves.
To give a basic description of the premise, five teenagers stumble upon a crashed alien spacecraft. The dying alien they find inside tells them of the Yeerks, a parasitic alien race that have been steadily infiltrating human society in order to take them over. The dying alien grants them the power to acquire and transform into any animal, so they might defend themselves, and hopefully hold the invasion force back, at least for a while.
After being off the shelves for a decade, Animorphs was brought back into print just this year. The first book was re-released last May. Sadly, they gave it a really terrible cover and had the gall to remove the little flipbook gimmick in the lower right page corners because apparently there is nothing sacred left in this world.
In case your memories of the nineties are a little hazy, here's a tiny character sheet of the five main humans of the series. You can also go to the much better tvtropes character sheet, but just know that if you click the link, you'll be on that website for hours. It's a science fact.
Jake: The leader of the group. He is saddled with a lot of responsibility, and is usually the one who makes the group miss fun activities to go on stakeouts and stuff. He gets paired up with Cassie pretty early on. Rachel is his cousin, so you're not supposed to ship them, you sicko.
Rachel: I believe finding out about the Yeerks was the best thing that happened to Rachel. Now all that murderous rage has an acceptable target. She also enjoys shopping and gymnastics.
Cassie: Cassie is actually the go-to girl for new morphs because her parents work at a wildlife rehabilitation clinic AND a zoo. That was convenient. She's pretty much Rachel's opposite in terms of personality and color scheme, and she tries to be a pacifist.
Marco: Smartass punk who uses heavy sarcasm to mask his fears and insecurities. Came up with the name "Animorphs" and often feels like the only sane person of the group. Poor Marco never got a consistent love interest, but he makes up for it by flirting with everyone.
Tobias: Strange loner kid who spends most of the series as a red-tailed hawk. He is very angsty, even moreso than the other characters, but he gets the coolest best friend ever later on. Romantically, he is paired with Rachel.