Try to find somenthing that is a little bit above your current level. You're right that you won't progress as fast if you stick to material you can easily understand, and that you'll also have a difficult time if you choose something too difficult. Ideally, you find the sweet spot in the middle, where you don't understand everything, you're slowly figuring out the meaning of new words and expressions, but you understand enough to follow along. It can be hard to find just the right level of material (that is also interesting to you!). You're the best judge of your current level, so you may have to just jump into interesting-looking books at random and see how well you do. If a book is too hard, save it for when you are more advanced.