Resources for learning intermediate to advanced 2D graphics resources?

What are some good resources to learn intermediate to advanced 2D graphics programming? Books would be preferable but suggestions for good websites would also be great.

Our background - we've released a couple of 2D RPGs using XNA & C#. My partner has some great pixel art skills but the 2D graphics engine I've programmed to support that pixel art is about as simple as it comes. Take target graphic asset and draw it at coordinates x,y. Maybe do some scaling and the occasional transparency. Very simple stuff.

Whereas the visuals for our games are high on art assets and low on programming, you also see some games that are low on art assets but still manage to look very cool through their special effects (like your typical Geometry Wars-inspired dual-stick shmup). I obviously have a lot to learn when it comes to programming 2D graphics and effects.

The question is what's the best way to learn that aspect of programming? Most programming books on graphics are focused on 3D games. And books that are focused on making 2D games are usually intended for beginners and focus more on the making games aspect and less on the 2D graphics.

Suggestions?