The First Aid for Horses is a small spiral bound book that you can throw in a tack box or medical kit.
I was thinking you were a nurse and might find something like the Merck Manual interesting and also the vet books.
G'Morning Duchess, All!
I'm catching up here so pardon me for posting a late suggestion about your horses eating each other's tails.
I can't remember how or if you feed hay to your horses, but keeping a roll of hay out there with them all the time might keep them occupied enough to keep them from doing this. It sounds like a stable vice they've picked up, kinda like cribbing, and they'll probably keep doing it unless you give them something else to do.
It's been my experience that horses just need something to chew on all the time to give them something to do, and if they don't have grass to chew on, they're GONNA find something else, be it a post, a tree, a rail or a tail. I think God designed them that way.