"Älykkään ritarin Don Quijote de la Manchan elämänvaiheet" by Miguel de Cervantes is a Spanish novel published in two parts in 1605 and 1615. A country gentleman reads so many chivalric romances that he loses his sanity and decides to become a wandering knight. Calling himself Don Quixote, he recruits a simple farmer, Sancho Panza, as his squire. Together they embark on adventures where windmills become giants and inns transform into castles, as Don Quixote's illusions clash with reality in what's considered the first modern novel.
Wikipedia page about this book: en.wikipedia.org/wiki/Don_Quixote Wikipedia page about this book: fi.wikipedia.org/wiki/Don_Quijote