|Madrid for kids|
There are many Illuminated manuscripts open at beautiful pages. Look at the designs on each of them. If you have a camera, take a photo of the manuscript you think is the most beautiful. Some pages have gold leaf. Before the Gutenberg Printing Press  books were copied mainly in monasteries, or (from the 13th century) in commercial scriptoria, where scribes wrote them out by hand.
Books were therefore a scarce and precious resource. It might take someone a year or more to hand copy a Bible. At El Escorial there have been a number of fires and luckily the main library has been spared but some collections stored in other areas have been lost.
Notice how the first letter of the first word on the page is often very ornate and much larger. There is also often a beautiful border. The Illuminator may incorporate flowers or people into the primary letter.