A little bit of both. Have one where you can understand 80%-100% then try to get one where your goal is to reach 80% comprehension. Reading challenging books will help you find new vocabulary and face new grammar structures, but lower level stuff will give you the opportunity to see it in action without having to look up every other word.
read material at-level or above-level? i'm about 30-40% A1 russian
A little bit of both. Have one where you can understand 80%-100% then try to get one where your goal is to reach 80% comprehension. Reading challenging books will help you find new vocabulary and face new grammar structures, but lower level stuff will give you the opportunity to see it in action without having to look up every other word.