Monkey is a religious reference in Book One (Awake to Emptiness) in The Years of Rice and Salt.

According to http://www.godchecker.com/pantheon/chinese-mythology.php?deity=MONKEY&ds=N

From the beginning of time, a certain rock on the Mountain of Fruit and Flowers had been soaking up the goodness of nature and QI energy. One day this pregnant rock released a stone egg, and from it hatched a Stone Ape, who solemnly bowed to the Four Corners of the Earth — then jumped off to have fun.

This was MONKEY. He was high-spirited, egotistical and full of mischievous pranks. It wasn't long before reports of MONKEY's tricks started to reach the austere ears of the JADE-EMPEROR. First the DRAGON-KINGS complained of rudeness and theft. Then YEN-LO-WANG, the God of Death, lodged a formal protest. The Jade-Emperor gave Monkey a job in heaven to keep him occupied. But before long Monkey was up to his old tricks again. Now the JADE-EMPEROR finally lost his esteemed cool and called in the Heavenly army to obliterate Monkey once and for all but they wound up making an embarassing display. There was nothing for it — the Ruler of Heaven called for BUDDHA.

Now BUDDHA, in his infinite wisdom, knew better than to subdue MONKEY by force. Instead he offered him a wager. "If you're so clever, jump off the palm of my hand. If you can do that, I'll take the Emperor in as a lodger and give Heaven to you. But if you can't, I'll expect a full apology and penance."

The Monkey King laughed to himself. He could travel thousands of miles in a single leap. The bet was on. BUDDHA stretched out his hand and MONKEY jumped...

Several thousand miles later, the Great Sage landed in a desolate plain with great columns reaching up the sky. "These must be the Five Pillars of Wisdom at the end of the Universe", he thought. "That BUDDHA is just plain stupid to make such a silly bet." And, to show his disrespect, he pissed all over the nearest pillar and jumped back to claim his reward.

"Is the Emperor packing his bags yet?" asked MONKEY as he landed. The Holy One raised a sublime eyebrow. "I don't know why you're grinning," he said, "you've been on my palm the whole time. Look." An astonished MONKEY rubbed his eyes and stared at the five familiar-looking pink pillars of BUDDHA's hand. Then he smelt the stench of monkey pee and trembled. The next thing he knew, he was lying on the ground with a mountain on top of him.

And there he stayed for five hundred long years, being fed molten copper and iron pills by an attendent demon while the moss grew in his ears. By the time GUAN-YIN came along, the Great Sage Equal of Heaven was a thoroughly humble creature.

As told in Journey To The West, GUAN-YIN enlisted MONKEY as chief disciple of the young Buddhist monk TRIPITAKA. Together with SANDY and PIGSY, he protected the boy on his quest to India, battling demons and righting wrongs along the way. His natural monkey trickery now had a holy purpose which he unleashed with much enthusiasm — and his uncontrollable ego was kept firmly in place by a little device of GUAN-YIN's devising: a head-band made of gold.

The unsuspecting Great Sage was not prepared for the terrible torture of the Headache Sutra! Whenever MONKEY misbehaved, TRIPITAKA recited the Sutra and the golden fillet squeezed until his very eyeballs felt like bursting. Try as he might, he could not remove it. There was no defense except submission, and pretty soon MONKEY was the most humble disciple the world has ever known. Usually.

After many many many many adventures, the travellers fulfilled their quest. MONKEY was rewarded for all his efforts with the title 'Buddha Victorious Against Disaster' and finally made his peace with Heaven. We don't know what the Great Sage gets up to nowadays, but presumably he keeps himself occupied.

References
Monkey is referenced in Book One (Awake to Emptiness ) of The Years of Rice and Salt on page 3. Robinson references Monkey to tie in Chinese mythology with historical references. He ties Monkey, who turned his trickery into a holy purpose and protected Tripitika on his journey to India to deliver scriptures, and credits him with being reincarnated as Bold to hep Temur the Lame in his military battles and conquest.