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
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