Anything from Western Horseman publications is probably helpful for you. Richard Shrake’s Western Horsemanship in that series is an oldie but goodie and great starting point, and their Guide to Western Tack is also good.
Note that western riding is a style of riding, but there is also a horse show class called western riding which is a very specific event that combines cadenced lead changes with a bit of obstacle and agility work. You’re looking for the general, not the specific.
You might enjoy following some of the blog/websites of western horse tamers…take it all with a grain of salt though. Many of them are good teachers, just trust your basic horsemanship skills to not get seduced into thinking there’s anything tricky about western riding. Mostly… the saddle, the hats, and the boots! I don’t think horses care too much.
Check out Stacy Westfall too, she has some great new free podcasts that really get inside the mind of the thoughtful equestrian. www.stacywestfall.com
Happy Trails!