With what one senior official calls "chilling clarity," the letter outlines al Qaeda's strategy for Iraq and beyond, reports CBS News national security correspondent David Martin.
The letter, which was written shortly after the London bombings in July, calls Iraq "the place for the greatest battle of Islam in this era."
Zarqawi is America's most-wanted insurgent in Iraq. Zawahiri, the man most intelligence analysts believe is the brains behind bin Laden and his al Qaeda terrorist organization, is considered the world's second most wanted man.
As outlined by Zawahiri in the letter, al Qaeda's battle plan calls for driving the Americans out of Iraq, establishing an Islamic regime in as much of the country as possible and then extending the holy war to other Arab countries, including Egypt, Lebanon and Syria.
In the letter, Zawahiri complains to Zarqawi that some of his violent tactics are hurting public support for al Qaeda's cause, particularly the videotaped beheadings of hostages.
"We don't need this," the letter says. "Use a bullet instead."
Zawahiri also complains about Zarqawi's all-out war against the Shiites of Iraq, saying the Arab man in the street doesn't understand why suicide bombings are killing so many fellow Muslims.
The letter also indicates Zawahiri's life in hiding has left him cut off from news and financial support. He asks Zarqawi to provide him more information about operations in Iraq, saying he should know at least as much as the enemy knows, and he even asks Zarqawi to send money.
The letter is 6,300 words long and a senior official calls it the clearest and most comprehensive exposition of al Qaeda's objectives and strategy the United States has ever seen.