If I could own only one book on riding, it would be:
The second half is the US Cavalry Manual from the 1940s. It can be a hard read at times because it is hard to describe on paper what a person does on horseback, but it goes into detail on how to cue your horse, the various aids available, etc. It is written from an English saddle perspective, but it isn't hard to translate some of that to a western horse/saddle. And like any Army manual I've seen, it is very much a "do this" approach...