Hell to Pay: Operation DOWNFALL and the Invasion of Japan, 1945-1947 by D.M. Giangreco
Honestly, it would have been hellish. Not so much from an invasion stand point per say, but the fact that the US and it's allies considered dropping as many as 9 additional atomic bombs.
Also (on edit) here's a clip from Wikipedia...
http://en.wikipedia.org/wiki/Operation_Downfall#Nuclear_weaponsNuclear weapons
On Marshall's orders, Major General John E. Hull looked into the tactical use of nuclear weapons for the invasion of the Japanese home islands (even after the dropping of two strategic atomic bombs on Japan, Marshall did not think that the Japanese would capitulate immediately).
Colonel Lyle E. Seeman reported that at least seven bombs would be available by X-Day, which could be dropped on defending forces. Seeman advised that American troops not enter an area hit by a bomb for "at least 48 hours"; the risk of fallout was not well understood, and such a short amount of time after detonation would have resulted in substantial radiation exposure for the American troops.<34>
Ken Nichols, the District Engineer of the Manhattan Engineer District, wrote that at the beginning of August 1945, "
lanning for the invasion of the main Japanese home islands had reached its final stages, and if the landings actually took place, we might supply about fifteen atomic bombs to support the troops." <35> An air burst 1,800–2,000 ft (550–610 m) above the ground had been chosen for the (Hiroshima) bomb to achieve maximum blast effects, and to minimize residual radiation on the ground as it was hoped that American troops would soon occupy the city.<36>