Playing in the pocket is when you have REALLY good timing and everything is really tight and together.
Pocket is not timing. Pocket is the perfect groove spot at a given tempo for a song. It can be in front of the beat/click, dead on top of the beat/click, or slightly behind the beat/click. This pocket is not off in tempo but it is not stiff or bound by the exactness of the click. Most people, Dennis Chambers, Steve Gadd defie it as playing slightly behind the beat. You have to be in tune with your inner clock and develop you inner clock with a metronome click not with a loop. You have got to know how to play all aspects of time so that you can navigate time.
***(Read my post on Stiff Playing or Too much pocket - Which has anexcert from the Modern Drummer Magazine interview with Micheal White whose nickname in the industry is Pocket Monster)
Russ Miller DVD: Drumset Crash Course
Steve Smith DVD: Drumset Technique and the U.S. Beat
Dave Weckl DVD: How To Develop Technique
Benard Purdie VHS: Groove Master
Jeff Porcaro DVD: Star Licks
J.R. Robinson DVD: Star Licks
Steve Gadd: Up Close
Steve Gadd: Sessions
Zoro: Commandments of RnB Drumming
Tommy Igoe DVD: Groove Essentials
Learn to read music and get these books:
Master Studies by Joe Morello
Patterns by Gary Chaffee
Single Surface Technique by Roy Burns (I'll have to check that title and confirm)
Advanced Funk Studies by Rick Latham
Contemporay Drumset Technique by Rick Latham
New Breed by Gary Chester