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

http://wiki.polycount.net/3DTutorials/Modeling%20High-Low%20Poly%20Models%20for%20Next%20Gen%20Games?action=show&redirect=3D_Tutorials%2FModeling_High-Low_Poly_Models_for_Next_Gen_Games

http://www.bencloward.com/tutorials_normal_maps1.shtml