In the first sentence we are expressing an action in which there is no spacific time sequence ( it means we dont know exactly when it happen ) But we know it is a continuing action and it maybe repetitive beside that there is possibility of happening in the future as well. In the second sentence the action was in progress for some time but it is finished now. In the last sentence, it is general fact about someone. He or she reads a lot.

You have been reading too many books. > "Have been" indicates an action taking place in the past lasting untill now ( present pecfect continous tense) > You were doing it in the past ( not specified in your sentence since when ) and you are still doing it. You were reading too many books. > Sometime in the past , you were doing this but not now since you stopped that in the past too.) >Past tense You read too many books.> This is what you do now >Present tense You have read too many books. > You read a lot of book in the past until now .> Present perfect tense) , whether you will do it in the future is not certain. Too many books > has a neagative conontation , meaning " overdoing" your reading of books.