The answer to this can be found in Chapter 3. It is on pages 78 and 79 in the paperback edition. The answer can be found in the part where Diamond is telling us why the Spanish came to South America instead of having the Inca come to Europe.
Diamond argues that literacy was a major factor in why the Spanish were able to cross the ocean while the Inca were not. Diamond says that literacy allowed information to "be spread far more widely, more accurately, and in more detail" than was possible for any society that did not have writing. The accounts of Columbus's voyages could be published, making people aware of the New World and eager to go there.
At the same time, the Inca did not have writing. This helps explain why they walked into a trap that Pizarro set. The Inca who met the Spanish were not able to send accurate and detailed reports to the government. This made the government much less aware of what the Spanish were capable of doing.
These were the major reasons why literacy was important in causing the Spanish to be able to overthrow the Inca.