Probably you want to try with a book you already know in English, so as you read you'll recognize the situations and language will be easier to understand.
If you're a Harry Potter fan, they're written in Spanish with the same basic language as they are in English.
Or try editions for the young public (teenagers and young adults); those are usually written in plain styles without much vocabulary.
However, I strongly suggest that you open any book you're attracted to; if you like the topic, it won't be hard to follow anyway.
