Choosing reading matter to help you learn English is difficult. The most important thing is that you read something which interests you. For one person that might mean reading romantic novels, for someone else maybe reading photography magazines is a better idea. I wouldn’t recommend starting by reading books aimed at young children; the language may be simpler, but that advantage is offset be the fact that the vocabulary won’t be as useful (unless you really want to learn childish expressions or about fairies, evil witches and so on!).

Books for young adults are often a good place to start – the language is still a little easier to understand, and the variety of literature available is similar to adult literature. Don’t try to start reading a 500-page book for your first attempt, better to choose books which are 100 – 250 pages long, so that you’re not stuck reading the same book for too long. If you think you might like reading a book, check its page on amazon.com, and you can see a description of the book, the number of pages, sometimes even read a few pages of it, and read what other people think about it.

If you are lucky enough to own a Kindle with a touch-screen (not a Kindle Fire, as they don’t support using the dictionary in the same way) you can read books a slightly higher level than you would otherwise, because you can use the built-in dictionary to translate words between your language and English. It’s much more convenient than using a paper dictionary and your English will improve more rapidly as a result.

Below are some ideas, in no particular order, to help you find texts to improve your reading skills:

SourceLevelComments
linguapress.comBeginner / Intermediate / AdvancedInteresting short texts accompanied by exercises based on the the text, grouped according to the student's level of English.
Charlie and the Chocolate Factory by Roald Dahl IntermediateAlthough this is a children's book, it is such fun that I have to recommend it. I read it at least three times in Spanish to improve my Spanish.

Anything else by Roald Dahl is also worth a try.
City College of San Francisco: Novels for Intermediate Level ESL StudentsIntermediateWhat's really good about this list is that it covers a variety of topics, so most students can find something interesting at their level.
Go ask Alice

Available from Amazon Spain here.
IntermediateA 1971 novel about the life of a troubled teenage girl, written in the form of the diary of an anonymous teenage girl who becomes addicted to drugs.

The novel's title was taken from a line in the 1967 Grace Slick-penned Jefferson Airplane song "White Rabbit" ("go ask Alice/when she's ten feet tall").
Listopia Best Young Adult BooksIntermediateReading books for young adults can be interesting for adults too.

This list includes books and series of books which I have enjoyed a lot as an adult and which have been turned into Hollywood films, such as the Hunger Games series, the Harry Potter series, The Maze Runner, The Book Thief, The Hobbit and many more.
ESL: Reading English at the Intermediate LevelIntermediateLucy Tse has some recommendations in her blog entry. She recommends books from the winner's list of the Newbury Medal awards. These are books especially chosen for children, but many are quite sophisticated and will appeal to adults too.