Mid 90s they came out and were super popular. At all the school book fairs. There's about 60 books in total and each one is about 200 pages. They were probably the most read book among kids and teens before Harry Potter, so chances are you weren't born soon enough to know much of them

As far as YA books go, they were brutal as hell. Lots of killing, enslavement, difficult moral choices, sacrifices, and fucked up situations. Without giving too much away, it's about a group of teens that get powers given to them by a dying alien that's part of a huge war with another alien race that's wanting to enslave/control the galaxy. They are given the power to temporarily change into any creature that they have touched. No other humans on the planet who haven't been enslaved and controlled by the bad alien species (that crawl into your ear and enslave you, taking over your mind) is aware of aliens existing, and the teens are left trying to hold off the bad aliens long enough for more of the "good" aliens to get to earth and help.

If you sail the high seas you can find all the books in e reader format, or if you'd prefer, a few years ago they made audiobooks of the entire series.

All the book covers look a bit cheesy and they made a tv show that was horrible due to cheap budget and having to make it rated G. I don't know what nickelodeon was even thinking trying to make it into a kids show. No chance that was going to work out well.

So you get those cheesy looking covers and then inside the books you get descriptions of a girl turning into a grizzly bear and ripping off heads while getting covered in blood while being shot at by alien slaves that are being ripped apart simply because they were enslaved and controlled by alien slugs that have taken over their minds.