OVERVIEW
http://tech-artists.org/wiki/Normal_map
http://wiki.polycount.com/NormalMap?action=show&redirect=Normal+Map
TOOLS
http://developer.nvidia.com/nvidia-texture-tools-adobe-photoshop
http://www.crazybump.com/
TUTORIALS
Transferring detail from high-res model to low-poly model in Maya
http://www.3dtutorialzone.com/tutorial?id=48
http://www.katsbits.com/tutorials/textures/how-not-to-make-normal-maps-from-photos-or-images.php