But not every object of okumak is definite. We have had many examples of indefinite objects of okumak in the chapter that introduced the definite object case. And this optative or imperative "let's" form doesn't make all objects definite, does it?
Also, "Biz hangi gazeteyi okuyalım?" would suggestthat there are other people reading too and "we" are deciding on which newspaper to read as putting an additional 'biz' would put the emphasis on "biz" instead of "hangi". I hope it is not very confusing.