"Don Quijote de la Mancha" by Miguel de Cervantes Saavedra is a Spanish novel published in two parts in 1605 and 1615. A minor nobleman reads so many chivalric romances that he loses his sanity and decides to become a wandering knight. Renaming himself Don Quixote, he recruits a simple farm laborer, Sancho Panza, as his squire. Together they embark on adventures where Don Quixote's delusions transform windmills into giants and inns into castles, blurring the line between fantasy and reality in this founding work of modern literature.
Wikipedia page about this book: en.wikipedia.org/wiki/Don_Quixote