"Are you reading books?"
Translation:Liest du Bücher?
It's incorrect with "bist". Explanation: it's incorrect, because in German you don't have all the tenses what in English, the tense-system is much more simple, for example here is a list of all the possible declinations of the verb "lesen": https://de.wiktionary.org/wiki/Flexion:lesen . That does not mean that every one of these is used (how many times do you use future perfect continuous? Whenever you are going to have seen the movie, right?)