The best way to learn racquetball is with in-person coaching.
A few coaches have provided fliers with their rates and availability:
Scott McClellan Lessons Flyer
Mike Bradford Austin Racquetball Coach Flier
The best professional racquetball player in the history of the game, Kane Waselenchuk, also provides lessons and clinics via his website.
There are a few players listed with USA Racquetball and certified via their Instructors program.
You can find those here:
AmPRO-iPRO also has a certification program.
You can find their instructors here:
There are also a number of professional players and coaches that can work with you via video. (i.e. you record video of playing or doing a drill, send it to them, they critique it and tell you how to get better.)
Pro Racquetball Academy
Adam Manilla, Erika Manilla & Bobby Horn – AKA Manilla Athletics