Horses are actually surprisingly delicate for being such large and strong animals.
I had quite a few lessons when I was a kid and I was obsessed with horse books as well. I remember one day the horse I was riding came down with a colic. She began to sweat heavily and got all lathered.
The woman insisted I keep riding her, since she needed to be kept moving, but the poor thing kept trying to sit down.
She also made sure we always walked our horses around for a bit after trotting, so that they would cool down. It's definitely a good habit to get into.
I remember reading about a horse being put away hot in Black Beauty and how it ruined the horse for life and I really took that part of my lessons to heart. I think every young rider should be made to read that particular book, it has a lot of good advice about horses.