+1 to "Understanding exposure." Beyond that, it does depend on what sort of pictures you want to learn how to take. For example, if you're into nature/landscapes, I
highly recommend any of John Shaw's books.
Another example would be to study the masters. I shoot a lot of black and white so my bookshelf, in addition to the "how-to" guides mentioned above, is filled with collections of coffee-table-style books featuring the work of folks like Robert Doisneau and Henri Cartier-Bresson.
One more recommendation on a Web site is the online community
www.nikonians.org. Excellent place to learn technique, receive critiques, and ask technical questions specific to your gear.
Hope that helps!
Jason
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E90 335i, black sapphire metallic, terra dakota, PP, SP, 6MT, cold weather, iPod, CA, burl walnut, LeatherZ ZHP shift knob, 19" M5 reps w/ Kumho Ecsta SPT staggered, BMC drop-in, SSTT. Amen.