Here's a few GREAT books I have... Not all of them are entirely about colts, but all about training and young horses, etc.
Starting Colts by Mike Kevil-- This one's definitely the best for info about young horses, overall.
World Class Reining
Reining with Al Dunning
And last, but not least...
Barrel Racing w/ Charmayne James
And again, most of these are specialized... But they are generally about working/starting these with younger horses, and they have some good info. I'm pretty sure you're not big on reining or barrel racing, but if you ever just got a chance to look through those books, they're really helpful.
But overall, the Starting Colts one is the best for what you're looking for. It's a spectacular book.
There's also a few other Western Horseman books like these, but leaning more toward weanlings/yearlings and the young horse... But I can't recall the names.
I'm sure if you googled a list of them, though, you could find them. Hope this helps!