There are a huge number of resources available to help you learn more about playing racquetball. Books, videos, and trainers that can work with you via digitally videoed sessions are all possibilities. Local instruction would be my first recommendation though. Nothing compares to the live feedback of someone that knows what they are doing and what to look for, right there in the moment.
Videos – Online
Racquetball Warehouse has produced and hosts some of the best, short, instructional videos. I recommend them.
Malia Bailey did an intro series with MonkeySee that is a good first step for brand new players.
Inside the 209 has 20-30 minute videos that feature great players and coaches from Stockton, California.
RacquetWorld produced a set of videos with Jim Winterton and a new series with Rocky Carson.
Pacific Sports Warehouse (now part of RacquetWorld) has a series of videos with Aaron Embry.
Royster Racquetball produced some videos with Cliff Swain.
RacquetballTim does a great job examining game play and making recommendations to improve the game of those being critiqued.
The Racquetball Practice Program put out some videos explaining good ways to practice shots.
Superior Stringing Services put out some off court racquetball exercise recommendations.
Kim Roy Hardison did a series of videos with LiveStrong.com.
RB Lessons created a series of playlists featuring many of the sources above.
Championship Racquetball by Fran Davis and Jason Mannino (on Amazon)
Winning Racquetball by Ed Turner and Woody Clause (on Amazon)
Advanced Racquetball by Steve Bo Keeley (on Amazon)
There are lots of others to read if you want alternate viewpoints. Search on Amazon.
Videos – DVD
Building Your Racquetball Dream House – by Fran Davis, featuring Jason Mannino and Sudsy Monchik
Beginning Racquetball: Skills and Drills by Jim Winterton
Advanced Racquetball Secrets of the Pros! by Jim Winterton
Secrets of Power Racquetball: Mastering the Basics by Marty Hogan
Secrets of Power Racquetball: Tips for Advanced Players by Marty Hogan
Play Better Racquetball: Skills and Drills by Shane Vanderson
Play Better Racquetball: Strategies by Shane Vanderson