English word order: libellus quem recitas est meus, o Fidentine

Relative pronouns do not necessarily agree in case with the nouns they modify.  Rather, their endings depend upon their functions in their relative clauses.  Here, quem does not agree with libellus in case (quem is accusative singular and libellus is nominative singular), but it does modify libellus.  Quem is in the accusative because it is the direct object of recitas.   The main verb in this line is est.  Recitas is a subordinate verb, which belongs to the relative clause quem recitas.