The books themselves don't have a formal, separate guide, but the herbal knowledge is scattered throughout the series. Characters like Yellowfang, Cinderpelt, and Jayfeather are always rattling off uses. It's the kind of thing you pick up over many books.
I actually made my own list once. Common ones are catmint (or catnip) for greencough, poppy seeds to make a cat sleep and ease pain, juniper berries for bellyaches, and cobwebs to stop bleeding. There's also things like horsetail for infected wounds, and chervil for belly pain or kitting. I remember Goldenflower getting dock leaves for a sore paw once, too.
Some of it's obviously fictional for the world, like traveling herbs or deathberries. But a lot are real plants with real historical uses, which is a cool detail Erin Hunter included. Trying to remember them all is half the fun of a re-read.
I sometimes wonder if it'd be helpful for a real cat to eat some of these. Probably not, but my brain goes there.