I like instructional videos because you can place your kit in front of the TV and DVD player and with the remote repeatedly watch and practice the video until you catch onto the groove:
These are old, but available on Amazon.com:
Ginger Baker: Master Drum Technique has great studies on paradiddles and doubles for fills and also techniques on substitution
If you can understand and master his paradiddle/double techniques you will be a different drummer
I also like Rick Latham: Advanced Funk Studies - some great fill and rythm ideas and exercises
YouTube has an abundance of videos you can download and replay:
Obviously I enjoy studying paradiddles/doubles in fills - also check out Crazy Diddles On The Kit by Vanz Cool Drum Tips on YouTube
One thing about these guys that makes all the difference: their stroke patterns are C-L-E-A-N - that's the real secret