Animorphs
What It Is 
Animorphs is a children's book series from the 90s about children who obtain the ability to transform into animals as a means of defending Earth from a covert alien invasion.
While the series is infamous for its uncanny book covers and has plenty of juvenile or less-than-coherent plotlines, it contains some hard-hitting elements and scenes about war, morality, and trauma. All of the series – well over fifty books – is available for free online with the author's blessing.
What It's About
In the first book, five kids happen to walk home from school together and decide to take a shortcut through the junkyard, only to find a wounded blue centaur-like alien, an Andalite, who warns them their planet is in danger. Another race of aliens, the parasitic Yeerks, is targeting Earth in order to obtain more hosts for its growing empire, and in a desperate bid to give them a fighting chance, he uses Andalite technology to give them the ability to transform into any animal they can touch. Soon after, the kids flee as the Yeerks arrive and finish the Andalite off for good.
For the rest of the book series, the children work to sabatoge the Yeerk conquest through various means. As the story progresses, they're joined by a stranded young Andalite named Aximili, too.
Why I Like It
I won a library-wide children's reading contest by reading all of Animorphs throughout the summer, so I may be biased. I found the writing both silly and intriguing. While there was definitely a lot of filler, the events of a previous story would rarely just "reset" at the end of the book. There was a sense of progression and change, even if it was slow. Besides, being able to turn into animals would be very cool, so reading about kids that could do that was fun.
There's books about breaking into Area 51 for an alien toilet, sure, but there were a lot of mature stories, too. The David arc is amazingly dark for a kids' book, as are the overarching plotlines revolving around Marco's family, Jake's guilt, Tobias's identity, Cassie's moral plight, Rachel's ultimate fate, and Ax's feelings of alienation. It gets you interested in the fun aliens and animal powers, and then it hits you with the message that there are no winners in a war.
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