Start with basic books but try to move up quickly to more challenging material. I read through Lord of the Rings in second grade. It was slow and frustrating at times, but the fact that the book is so engaging helped a lot. I learned so much vocabulary in such a short time this way.
Don't be afraid to keep a hard-copy dictionary of the language you're studying handy.
Don't be afraid to keep a hard-copy dictionary of the language you're studying handy.