Books you need for Naruto D20

Mandatory:

Player's Manual or D20 Modern

Both of these books provide much needed information, from explaining how to play

to everything your character needs. D20 Modern is more useful in the Feat and Action Point department, where Player's Manual has tons of weapons and armor examples and a better explination on the rules. If you have both, it's for the best.

DM's Guide

For the GM to use, because it has the rest of the rules.

Optional Books:

Arms and Equipment

This book has everything a ninja would ever want.

Stronghold Builder's Guide

This is useful if anyone goes for the Village Victory or at least wants a mansion for their clan.

D20 Past

This is mandatory if you're using the Multiverse Campaign Setting.

Any of the environment books

Sandstorm, Stormwreck, Frostburn, and whatever they come out with are good if you're into survival.

All of these books can be found on http://www.amazon.com/