El Quijote is, as you know, one of the main works to understand Spanish Literature and culture. It's not as much about its literary quality, as it is about its historical role shaping many aspects of the Hispanic world.
After several centuries, the book has undergone lots of changes to adapt it to modern Spanish, but surprisingly, the original version is not difficult to understand if your level of Spanish is intermediate or advanced and you have a good dictionary with you for the more antique terms and references.
Why is the premise of the book interesting? It's about a man who reads too many medieval novels and has a sharp attack of nostalgia for a world that he will never be able to experience. He wants to adapt reality to his desires and starts changing everything in the real world to fit his agenda. A waitress becomes a noble lady he must fight for, a donkey becomes a magnificent stallion, windmills become giants he must fight and defeat to banish evil from the land, etc.
There are many examples of similar situations in which we see things blown out of proportion or adjusted to fit artificial constructions, so sometimes we must stop and ask ourselves: Is this reality or are we fighting monsters that are merely windmills?
The edition you see here is from the 19th century and has illustrations of Gustave Doré.