Here are the texts in which Joseph Ratzinger explicitly treats "relativism."
*Truth and Tolerance: Christian Belief and World Religions especially "Relativism: The Central Problem for Faith Today" (address given in May 1996, Guadalajara),
*Preface to the New Edition of Introduction to Christianity "Yesterday, Today, Tomorrow," 2000
*Presentation of Dominus Iesus (2000)
*Without Roots: The West, Relativism, Christianity, Islam, 2006 (2004 dialogue with Marcello Pera)
*Homily at Mass Pro eligendo romano pontifice, 2005.
*Truth and Tolerance: Christian Belief and World Religions especially "Relativism: The Central Problem for Faith Today" (address given in May 1996, Guadalajara),
*Preface to the New Edition of Introduction to Christianity "Yesterday, Today, Tomorrow," 2000
*Presentation of Dominus Iesus (2000)
*Without Roots: The West, Relativism, Christianity, Islam, 2006 (2004 dialogue with Marcello Pera)
*Homily at Mass Pro eligendo romano pontifice, 2005.
Cf. The Dictatorship of Relativism, Gediminas T. Jankunas, New York: St Paul, 2011.
N.B. The only full length book by Ratzinger in that list is Truth and Tolerance, and even that, as with almost all of his books, is composed of a compilation of articles from various sources. Here are the titles and the contexts of those articles which comprise each of the chapters of that book.
I.1. "Der Christliche Glaube und die Weltreligionen," 1964 (on the occasion of the 60th birthday of Karl Rahner). 15-54
2. "Der Christliche Glaube vor der Herausforderung der Kulturen," Salzburg 1992. 55-109
II.1. "Relativism, the Central Problem for Faith Today," Address to the Presidents of the Doctrinal Commissions of the Bishops' Conferences of Latin America, Guadalajara, Mexico, May 1996. 115-137
2.1. "Glaube zwischen Vernunft und Gefühl," Hamburg, 1998. 138-161
2. "Verité du Christianisme," Sorbonne, Parish, 27 November 1999. 162-183
3. "Die Einheit des Glaubens und die Vielfalt der Kulturen," Introduction to the Encyclical Fides et Ratio, 1999. 183-209
III.1. "Faith--Truth--Tolerance," Lugano and Naples 2002. 210-231
2. "Freedom and Truth," Padua 1995. 231-258