They were YA animal fantasy novels....but very sophisticated ones.
In fact, I think that was part of the reason why they went out of print...they were a -lot- deeper than, say, the current "Warriors" series is....but also a much more demanding.
My local libraries used to have copies of every book, but now they're not even in the system as "withdrawn from circulation" anymore.
Happily, Bell's finally written a new installment, and with that comes reprints of the entire series.
It's set on an alternate, prehistoric version of Earth...one in which a race of cheetah-like cats are the ones developing culture and technology.
Vivid, fully-dimensional characters and some very exciting yet thoughtful plots made the series one of my favorites, back in the day.