Did I mention I love the Animorphs books? They're pretty awesome. I would do YouTube reviews for them, but someone else is already doing that.
Animorphs is a series where our heroes can turn into any animal they want. Also, they need to stop aliens from taking over the world. The premise is interesting, the books are funny and entertaining, and it was basically the 1990's equivalent of The Hunger Games.
The series was rock solid, until about Book 30. Then the ghostwriters took over, and things got a little weird. We got books with premises like "Cassie visits Australia!", "Let's learn about human biology by going inside Marco's body!" and "Jake had a fight at a donut shop, so now he's in an alternate future!". There's also the "one ghostwriter was hired twice, so they wrote a direct sequel to their first book", which occasionally happens with the Nancy Drew series.
The ghostwriters basically wrote the series into the ground. The original author resumed control, and she decided to officially end the series with a ten-book story arc. It was super dramatic, and it was also written by ghostwriters. I only read the finale arc once, and I remember it being okay. Not as good as the opening arc to the series.
I mention the ghostwriter stuff, because I didn't learn about it until adulthood. It totally explains why I lost interest in the series, at that point in time. So, overall, I'd say I love the series, except for the half that was done by ghostwriters.