Short answers, pulled from the story.
The Clementinum is a historic complex of buildings in Prague covering 20,000 square metres, making it the second largest building complex in the city after Prague Castle. It houses the National Library of the Czech Republic and includes a celebrated Baroque library hall and an Astronomical Tower.
The Clementinum's history dates to the 11th century, when a chapel dedicated to Saint Clement stood on the site. A Dominican monastery followed in the medieval period, before the site was transformed into a Jesuit college in 1556.
In 1791, the Clementinum was the site of what is described as the first World's Fair. The industrial exhibition was held to celebrate the coronation of Leopold II as king of Bohemia and showcased the manufacturing sophistication of Czech lands.
In 2005, the Czech National Library received the UNESCO Jikji Prize, part of the Memory of the World programme, recognising the significance of the collections held at the Clementinum.
The National Library at the Clementinum holds Mozartiana, materials related to the astronomer Tycho Brahe, works connected to the educator Comenius, and historic examples of Czech literature.
Jorge Luis Borges mentions the Clementinum in his 1943 short story "The Secret Miracle." In the story, the protagonist dreams of the Clementinum, where librarians search through four hundred thousand books seeking the letter that contains God.