Sources on The Planned Invasion of Japan: Operations Olympic and Coronet

CIA document on the possible invasion of Japan
Alternatives to OLYMPIC. JWPC 397, 4 August 1945
Codename Downfall
"Directive for Operation OLYMPIC." JCS 1331/2, 14 May 1945 (3/2008)
"Directive for Operation OLYMPIC." JCS 1331/3, 25 May 1945. (3/2008)
Hell to PayJapanese Reaction to An Operation Against Southern Kyushu. JIC 191/7, 16 May 1945
Magic: Fear East Summary on Japanese Defensive Preparations Against an Invasion, War Department, Aug. 4, 1945
Operation Olympic, the Invasion of Japan
Surviving Armageddon
The Burning Mountain (fiction)
The Invasion of Japan
X-Day Japan: Front Line Reporting at the Greatest Invasion and the Dawn of Nuclear Warfare