I would suggest trying the first Harry Potter book, "Harry Potter and the Philosopher's Stone," by J. R. Rowling. They are not written in easy English but it is not too difficult, and as an (elderly) adult I found all of them enjoyable and engaging. And, of course, it is easy to find a Spanish translation if you get stuck.
Another suggestion is George Orwell's "Animal Farm," available at no cost through Project Gutenberg Australia:
George Orwell writes in a beautiful English prose style. It is direct, plain, and clear. "Animal Farm" is short. It is NOT intended for children. It is actually a political allegory about the Russian revolution and a betrayal of its ideals by Stalin, but I did not understand that when I read it at age 12, and I enjoyed it anyway.