I think it depends on what you like to read. If you read a lot of books about a certain subject, then you'll have a bigger vocabulary in that area. As another commenter said, you could read books designed for English learners. If you want to read books that a native speaker would read but can't understand a lot of English, you may be limited to children's books. I think there's a lot of children's literature that is quite good, though. Some classics are Charlotte's Web, The Giving Tree, The Phantom Tollbooth, and handful of books by Roald Dahl. I think those are all books that native speakers can read in primary school, but they have themes and plots that many adults would also be able to appreciate.

Have you tired using graded readers? They are books designed for learners of English and can be sorted by level (A1/A2/etc.). Most of them come with narration too so that you can practise listening as well. http://www.learnenglish.de/books/gradedreaders.html