Note: "David Copperfield" is a long book written in Victorian (1837-1901) English; it was assigned reading in my high school English class but we found it challenging. The Harry Potter books are wonderful, but they are long, too.
Tell us more about your tastes and what you're looking for! Are you looking for very correct written English, or something contemporary and colloquial?
I'm thinking about "Animal Farm" by George Orwell, written around 1947 or so. George Orwell was a wonderful prose stylist and considered by writers to be a model of good English prose, and yet his writing is crisp and easy to read. It is short. It is famous and it will be very easy to find a Spanish translation if you get stuck. It is political but I think the politics are old enough not to be a problem for anyone--it is a satire or criciticism of the Soviet Union. (Orwell was originally sympathetic too, then disillusioned by the Soviet Union). It's an allegory about the Russian Revolution. But when I read it as a teenager, I wasn't even aware of this. It is interesting and engaging just as a story. It's humorous, but actually... ummm.... uhhh.... OK, I cried at the part where the horse Boxer is ultimately betrayed by the pigs.
Read about it at http://es.wikipedia.org/wiki/Animal_Farm ("Rebelión en la granja").
You get read or download it online at no cost at Project Gutenberg Australia:
http://gutenberg.net.au/ebooks01/0100011h.html
Project Gutenberg, http://www.gutenberg.net , and Project Gutenberg Australia, are wonderful resources for classic (out-of-copyright) books.

I will recommend Reader's Digest. Like any good magazine, it's full of articles from a diverse range of topics, contains jokes, anedcodotes, and short stories. The language is vibrant and is niether too challending nor easy peasy. You'll enjoy every word of it.

"Wonder" by R. J. Palacio
It's very short and eay to understand and so sad!

This is from Amazon:
'My name is August. I won't describe what I look like. Whatever you're thinking, it's probably worse.'

Auggie wants to be an ordinary ten-year-old. He does ordinary things - eating ice cream, playing on his Xbox. He feels ordinary - inside. But ordinary kids don't make other ordinary kids run away screaming in playgrounds. Ordinary kids aren't stared at wherever they go.

Born with a terrible facial abnormality, Auggie has been home-schooled by his parents his whole life. Now, for the first time, he's being sent to a real school - and he's dreading it. All he wants is to be accepted - but can he convince his new classmates that he's just like them, underneath it all?

WONDER is a funny, frank, astonishingly moving debut to read in one sitting, pass on to others, and remember long after the final page.

"To Kill a Mockingbird," by Harper Lee, not short but not long. A staple of high school reading lists. Read about it at http://es.wikipedia.org/wiki/Matar_un_ruiseñor . Strange and wonderful, hard to describe, about a ten-year-old girl growing up in Alabama during the 1930s, and about an episode of racial injustice.

My wife and I both loved "The Art of Racing in the Rain," by Garth Stein. Not short, not long. The first chapter is at http://www.garthstein.com/arr/excerpts.php . It is, um, told from the point of view of a dogs, who are supposed to have human-like intelligence but can't make this evident to humans. Like some dogs in real life, this dog likes to have the TV set on, and the idea is that everything he has learned about humans has come from television. It's a "tearjerker" (an enjoyable sad book). You can probably tell whether or not you will like it from this single passage:

'“In Mongolia, when a dog dies, he is buried high in the hills so people cannot walk on his grave. The dog's master whispers into the dog's ear his wishes that the dog will return as a man in his next life. Then his tail is cut off and put beneath his head, and a piece of meat or fat is placed in his mouth to sustain his soul on its journey; before he is reincarnated, the dog's soul is freed to travel the land, to run across the high desert plains for as long as it would like.

I learned that from a program on the National Geographic channel, so I believe it is true. Not all dogs return as men, they say; only those who are ready.

I am ready.”'

David Copperfield