Much of this collection likely originated in the fabulous Library of Constantinople, which was destroyed during the Fourth Crusade. It's also said that he received scrolls rescued from the Library of Alexandria as part of his wife's dowry, and references to "Livy's history" indicate that he might have had the lost books of the Roman historian Livy, which could massively augment the knowledge we have of Ancient Rome. Or, if you don't care all that much about history, know that Ivan bound around 800 of these works in gold, and gold is shiny.
Ivan Fedorov, Peter Mstislavets
Ivan's grandson, Ivan "the terrible" IV, became grand prince in 1533, when he was just a toddler. And under his adult reign, the library vanished. One account says he wanted to hide the golden library from Russia's enemies, while another says he buried it deep to protect it in case of a fire. Whatever his reasoning, he died without ever recording where he'd put it all, and explorers have spent centuries since trying to find it.
It's likely located in the system of tunnels beneath the Kremlin, which sounds like it narrows the search down pretty far. But those tunnels go back 700 years, and navigating them today means a whole lot of excavation. 1925's attempt to dig through them produced no results, while a promising 1933 effort was hushed up by a paranoid Stalin, who feared attackers using the tunnels. Archaeologists have expanded their search to various villages Ivan might have used as a hiding place, but the Kremlin remains everyone's best guess. So if you ever visit Moscow, we suggest visiting the Kremlin with a pickax and some dynamite. If any guards object, just yell "I am a scholar!" while waving your arms wildly.