I think I understand. So 読んでいます usually needing an object as in 本 and grammatical connect it with を its not possible to simply respond with "Hiro is reading at the library" without specifying an object?
Exactly. As I wrote previously, unlike 読む, 読書（を）する doesn't require the direct object, so there is no problem with 彼は図書館で読書（を）しています. Actually, the construction of 読書（を）する is 書を読むことをする, i.e., "to do reading written things (= "to read a book/books" as a result)", thus, it includes the object in the verb itself.