You may feel free to do your own independent web search, but I recommend you use the following sites to complete your prereading research project for John Hersey's Hiroshima.  While all the listed sites are appropriate, the asterisked ones are especially informative.

Hiroshima Project
 
http://www.historyplace.com/
The History Place

http://www.dannen.com/decision/index.html
Atomic Bomb:  Decision

http://www.lclark.edu/~history/HIROSHIMA/
Hiroshima Archive

http://www.yale.edu/lawweb/avalon/abomb/mpmenu.htm
The Atomic Bombings of Hiroshima and Nagasaki  *

http://atomicarchive.com/
Atomic Archive  *

http://www.nhk.or.jp/nuclear/e/cont.htm
Nuclear Arms and the Human Race  *

http://atomicmuseum.com/tour/manhattanproject.cfm
National Atomic Museum - Manhattan Project  *

http://atomicmuseum.com/tour/decision.cfm
National Atomic Museum - Decision to Drop the Bomb  *

http://members.home.net/jeromej/atomic/
The Atomic Bomb  *

http://www.engr.utexas.edu/uer/past/fall97/manhattan/index.html
Manhattan Project  *

http://gi.grolier.com/wwii/wwii_atom.html
The Atomic Bomb

http://nuclearfiles.org/docs/lastact3.html
Nuclear Files Archive  *

http://nuclearfiles.org/docs/unit2.html
Nuclear Files Archive - The Decision  *

http://www.csi.ad.jp/ABOMB/index.html
A-Bomb WWW Museum  *