This series is the Harry Potter of cat books. I started reading these almost as a joke, but soon I was completely invested in this world. These cats were brought to life with personalities more vibrant and complex than many human characters. These stories are full of danger, friendship, forbidden love, betrayal, and self-discovery and it is all surprisingly believable for talking cats. I had physical reactions - crying, laughing, tightening in my chest - to this series. I would highly recommend it, as long as you can suspend your disbelief, put away your preconceived notions about "talking animal books," and just enjoy a good story. No, it's not ground-breaking stuff, but it's beautiful in its own way. It left me staring at my cats and feeling vaguely disappointed that they couldn't talk.
This one is my personal favorite, mainly because of the friendship between Fireheart and Graystripe.