I have read Animorphs books before, including the first one, but I have to admit that they are things I read mostly for convenience. But recently, I read Book #4, "The Message", because I found a copy (complete with holographic cover) at The Dollar Tree, and because it is Science-Fiction month.
A rather complete synopsis of the plot is seen above, so I will give a shorter description, and then talk about how the book feels, more than two decades later. The book begins when our newly created animorphs team of teenage friends start having eerie, prophetic dreams. Although someone confused about it at first, they go on a mission to find what turns out to be a submerged flying saucer, where a powerful ally from the "friendly" alien race of the Andalites is trapped. Through a series of dangerous adventures, they free their new ally.
As several people have mentioned, here and elsewhere, the Animorphs was a dark series, considering its audience. Even though descriptions of this book might sound fanciful, as we see whales fighting aliens, the writing manages to keep an air of tension about it. And despite the basic level of characterization, common for series books, where we have a leader, a shy girl, a funny guy, etc., I found enough character depth to keep it interesting.
But we are reviewing science-fiction, so lets talk about this book as science-fiction. Most, or all of the science-fiction doesn't make sense. The background is classic space opera: two star-spanning empires are at war. They have technology that is beyond all explanation, including a city-sized flying saucer apparently capable of faster than light travel. And the morphing technology breaks basic laws of biology and physics: where does that extra mass come and go? So it might be easy to dismiss the books as not being serious enough for science-fiction. But I found one serious and very timely concept in this book that I think explains a science-fiction concept very well: the idea of identity, and identity modification. The morphs are described as changing someone's mental status, subsuming someone's emotions, while augmenting others. When the group becomes dolphins, they become so playful and light-hearted that they are unable to remember their human goals. And I find it interesting that this concept of mood and identity alteration coincides with two contemporary events: the discovery of SSRI's like Prozac (back in the 1990s, "Prozac" wasn't just one of many SSRI's, but a cultural phenomenon in itself), and the slightly later rise of the internet as a common thing. As the saying went at the time, "On the internet, no one knows you are a dog", and this was literally true for our Animorphs. Beneath the standard world they lived in, our protagonists discover alterations in their basic sense of self.
While it might seem a bit far-fetched to describe the books to be about the malleability of identity, and how technology can alter our identities, often without us even being aware of it, I do think that this would have been in the milieu when the books were written, and it would have worked its way into the books. So while this book might seem like a standard and improbable space opera on its surface, I think it has a sophisticated science-fiction concept below that.