For those who are serious about researching Operation Downfall, the following is a good starting list of primary source documents:
Operation Olympic Documents
AFPAC Staff Study, Olympic Operation in Southern Kyushu, 28 May 1945, RG 165, NARA
AFPAC Operations Instructions No. 1/9, 3 August 1945, RG 338, Box 193,
Engineer Annex to USAFPAC Operations Instructions No. 1, 20 June 1945, RG 338, Box 193
CINCPAC Staff Study OLYMPIC, 18 June 1945, RG 218, NARA
CINCPAC Operations Plan, OLYMPIC, USMC Geographic File, Japan, Box 50, Folder B1-1, WNRC
Sixth Army Field Order No. 74, Troop List, 28 July 1945, Records of the Strategic Plans Division, Box 187, NHC
Memo, Colonel Elliott to General Krueger, 28 June 1945, Sub: Command Request for Construction Groups and Construction Battalions for Olympic, RG 338, Box 196
Sixth Army Engineer Section Plans and Operations, 1943 1945, WNRC
Troop List of Service Troops to Stage through Luzon, 19 July 1945, RG 338, Box 59, Sixth Army G-4 Decimal File, 1943 46, File 4, WNRC
Memo, Sixth Army Engineer to Chief of Staff, 3 June 1945, RG 338, Box 59, Sixth Army G-4 Decimal File, 1943 1946
Letters, Allied Air Forces to CINCAFPAC, 3 August 1945 and CINCAFPAC to CGFEAF, RG 338, Box 191, File No. 5
Amphibious Forces Pacific Fleet Operations Plan No. A11 45, 10 August 1945, NHC
Amphibious Corps Operation Plan, No. 1 45, 6 August 1945, USMC Geographic File, Japan, Box 52, WNRC
V Marine Amphibious Corps Operations Report, Occupation of Japan, Appendix 3 to Annex C, 30 November 1945, Marine Historical Centre, Washington DC;
I Corps Field Order, 4 August 1945, RG 94, Box 3089, File 201-3.9, WNRC
IX Corps Field Order No. 1, Operation OLYMPIC, 12 August 1945, RG 94, Box 4105, File 209-3.9, WNRC
IX Corps Report of Reconnaissance and Survey of Japanese Dispositions, Southern Kyushu (Operation OLYMPIC-MAJESTIC), 15 December 1945, RG 94, Box 4104, File 209-2.0, WNRC
XI Corps Staff Conferences on OLYMPIC, 6-9 July 1945, RG 94, Box 4159, File 2.11-0.5, WNRC
XI Corps Tentative Plan for AAA Employment for Operation OLYMPIC, 8 July 1945, RG 338, Box 17, Operations Reports and Related Records, 1944 46, WNRC
Basic Logistic Plan, OLYMPIC, in Fifth Air Force Logistical Plan for Operation OLYMPIC, US Air Force Historical Center, Bolling Air Force Base, Washington DC
Operation Coronet Documents
Appreciation and Plan for the Defeat of Japan, JWPC 46/5, 9 July 1943, RG 218, NARA
Outline Plan for the Invasion of the Kanto Plain, RG 218, CCS Honshu (7-19-44), NARA
AFPAC Staff Study CORONET, 15 August 1945, RG 165, NARA
Eighth Army, CORONET Operation, G-3 Plans (Invasion of Japan), May-June 1945, RG 407, Box 2836, WNRC
AFPAC, DOWNFALL, Strategic Plans for Operations in the Japanese Archipelago, 28 May 1945, RG 15, OPD 350.05, NARA
Staff Study of Cover and Deception Objectives for CORONET, JWPC 190/16, 26 July 1945, RG 218, NARA
Logistical Plan for the Invasion of the Kanto Plain, JLPC 47/10, 8 May 1945, RG 218, NARA
Memo, Marshall to Hull, 28 May 1945, Verifax 1193, Item 2288, Marshall Library;
Message, Hull to MacArthur, 29 May 1945, Verifax 1193, Item 2799, Marshall Library
Defensive Preparations in Japan, 2 August 1945, JIC 311, RG 218, NARA
Japanese Reaction to an Assault on the Kanto Plain (Tokyo) of Honshu, JIC 218/9, 10 July 1945, RG 218, NARA
Operations following Invasion of Kanto Plain (Broad Plans), JCS 1417, 10 July 1945, in CCS 381 POA (4-21-45), RG 218, NARA
Operations in Japan Following CORONET, JWPC 333/1, 26 July 1945, Records of the Joint Chiefs of Staff (microfilm edition)
This is a list of Secondary sources (both books and articles) that provide some very in-depth background on Japanese culture, industrial and military preparedness as applies to Operation Downfall:
Dennis Showalter, Storm Over the Pacific, in The Pacific War Companion (Oxford: Osprey Publications, 2005), 15.
Edward S. Miller, War Plan Orange: The US Strategy to Defeat Japan (Annapolis, MD: US Naval Institute Press, 1991), 44.
Iriye Akira, Power and Culture: The Japanese-American War 194145 (Cambridge, MA: Harvard University Press,1981), 669.
Peter J. Wooley, Geography & Japans Strategic Choices (Washington D.C.: Potomac Books, 2005), 1011.
John Toland, The Rising Sun: The Decline and Fall of the Japanese Empire (New York: Bantam Books, 1970),
EB Schumpeter, Ed., GC Allen, MS Gordon, The Industrialization of Japan and Manchukuo 19301940: Population, Raw Materials and Industry (New York: The MacMillan Company, 1940), 67679
Takekoshi Yosoburo, The Economic Aspects of the History of the Civilization of Japan Volume 3 (New York: The MacMillan Company, 1930), 277.
Takekoshi Yosoburo, The Economic Aspects of the History of the Civilization of Japan Volume 1 (New York: The MacMillan Company, 1930), 282.
Joseph DAutremer, The Japanese Empire and Its Economic Conditions (New York: Charles Scribners & Sons, 1910), 135.
GE and Denzil Baring Hubbard, Eastern Industrialization and Its Effect on the West (London: Oxford University Press, 1935), 2.
George Forty, Japanese Army Handbook 193945 (Sparkford, England: Sutton & Company, 2002), 95103;
Richard B. Frank, Downfall: The End of the Imperial Japanese Empire (New York: Penguin Group, 1999) This is the single book to own on Operation Downfall.
Okumiya Matsake and Horkioshi Jiro with Martin Cadin, Zero! (New York: EP Dutton & Company, 1956). Zero! is an excellent chronology of the deterioration of the Japanese Air Forces that should be taken in toto.
Jim Rearden, Kogas Zero, Invention & Technology 13 (Fall 1997): 61, 63.
David C. Isby, CA: Tactical Naval Warfare in the Pacific, 194143, Strategy and Tactics Magazine # 38, May 1973,
Robert B. Edgerton, Warriors of the Rising Sum: A History of the Japanese Military (New York: W.W. Norton and Company, 1997),
Ronald Spector, Eagle Against the Sun: The American War with Japan (New York: The Free Press, 1985),
Walter Boyne, The Influence of Air Power Upon History (New York: Pelican Publishing Company, 2003),
Suzuki Kanji, A Kamikazes Story, MHQ: Military History Quarterly 7, Number 3 (Spring 1995).
George Feifer, Tennozan (New York: Tichnor & Fields, 1992)